wfc3_exception_report.ipynb: fixed image rendering

This PR fixes an issue found in wfc3_exception_report.ipynb where the two images failed to render.

This notebook checklist has been made available to us by the Notebooks For All team. Its purpose is to serve as a guide for both the notebook author and the technical reviewer highlighting critical aspects to consider when striving to develop an accessible and effective notebook.

The First Cell

[ ] The title of the notebook in a first-level heading (eg. <h1> or # in markdown).
[ ] A brief description of the notebook.
[ ] A table of contents in an ordered list (1., 2., etc. in Markdown).
[ ] The author(s) and affiliation(s) (if relevant).
[ ] The date first published.
[ ] The date last edited (if relevant).
[ ] A link to the notebook's source(s) (if relevant).

The Rest of the Cells

[ ] There is only one H1 (# in Markdown) used in the notebook.
[ ] The notebook uses other heading tags in order (meaning it does not skip numbers).

Text

[ ] All link text is descriptive. It tells users where they will be taken if they open the link.
[ ] All acronyms are defined at least the first time they are used.
[ ] Field-specific/specialized terms are used when needed, but not excessively.

Code

[ ] Code sections are introduced and explained before they appear in the notebook. This can be fulfilled with a heading in a prior Markdown cell, a sentence preceding it, or a code comment in the code section.
[ ] Code has explanatory comments (if relevant). This is most important for long sections of code.
[ ] If the author has control over the syntax highlighting theme in the notebook, that theme has enough color contrast to be legible.
[ ] Code and code explanations focus on one task at a time. Unless comparison is the point of the notebook, only one method for completing the task is described at a time.

Images

[ ] All images (jpg, png, svgs) have an image description. This could be
- [ ] Alt text (an alt property)
- [ ] Empty alt text for decorative images/images meant to be skipped (an alt attribute with no value)
- [ ] Captions
- [ ] If no other options will work, the image is decribed in surrounding paragraphs.
[ ] Any text present in images exists in a text form outside of the image (this can be alt text, captions, or surrounding text.)

Visualizations

[ ] All visualizations have an image description. Review the previous section, Images, for more information on how to add it.
[ ] Visualization descriptions include
- [ ] The type of visualization (like bar chart, scatter plot, etc.)
- [ ] Title
- [ ] Axis labels and range
- [ ] Key or legend
- [ ] An explanation of the visualization's significance to the notebook (like the trend, an outlier in the data, what the author learned from it, etc.)
[ ] All visualizations and their parts have enough color contrast (color contrast checker) to be legible. Remember that transparent colors have lower contrast than their opaque versions.
[ ] All visualizations convey information with more visual cues than color coding. Use text labels, patterns, or icons alongside color to achieve this.
[ ] All visualizations have an additional way for notebook readers to access the information. Linking to the original data, including a table of the data in the same notebook, or sonifying the plot are all options.

spacetelescope / hst_notebooks

wfc3_exception_report.ipynb: fixed image rendering #148