Introduction to grid lesson: fixing flex-box card exercise link

Because

Fixing the faulty link on the flex box card exercise image in the intro to grid lesson

Updating the link associated with the image to the new repo address

Closes #29102

[x] I have thoroughly read and understand The Odin Project curriculum contributing guide
[x] The title of this PR follows the location of change: brief description of change format, e.g. Intro to HTML and CSS lesson: Fix link text
[x] The Because section summarizes the reason for this PR
[ ] The This PR section has a bullet point list describing the changes in this PR
[x] If this PR addresses an open issue, it is linked in the Issue section
[x] If any lesson files are included in this PR, they have been previewed with the Markdown preview tool to ensure it is formatted correctly
[x] If any lesson files are included in this PR, they follow the Layout Style Guide