Tiny typo fixes and improvement suggestion to code example in "Design Philosophy" doc

[alfredocarella]

BEFORE change:

• The nanoplot in the second example included more columns than described in the source note (i.e. days 3 through 9 plus "norm_I" and "norm_u" columns, instead of just days 3 through 8). AFTER change:
• The nanoplot includes days 3 through 9 only (without "norm_I" and "norm_u")
• The source note on table footer is adjusted accordingly

Additional tiny typos:

• Remove unused "import great_tables"
• Remove superfluous "the" in text paragraph (line 142)

Summary

Thank you for contributing to great_tables... this is great! To make this process easier for everyone, please explain the context and purpose of your contribution. Also, list the changes made to the existing code or documentation.

Related GitHub Issues and PRs

• Ref: #

Checklist

• [x] I understand and agree to the Code of Conduct.
• [x] I have followed the Style Guide for Python Code as best as possible for the submitted code.
• [x] I have added pytest unit tests for any new functionality.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.40%. Comparing base (f8e30ea) to head (1111030).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #401 +/- ## ======================================= Coverage 86.40% 86.40% ======================================= Files 42 42 Lines 4700 4700 ======================================= Hits 4061 4061 Misses 639 639 ```

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[machow]

@rich-iannone do you mind taking a look? Should be previewable here https://docs-preview-patch-1--gt-python.netlify.app/