Fix typo in documentation for PIPENV_CACHE_DIR

[gautambhat]

Thank you for contributing to Pipenv!

The issue

There is a small typo in the documentation where "it's" is incorrectly used instead of "its" in a comment.

This is not associated with an open issue, but it's a minor fix to improve clarity and correctness in the documentation.

The fix

This pull request corrects the typo by replacing:

#: Location for Pipenv to store it's package cache.

with

#: Location for Pipenv to store its package cache.

Since "it's" is a contraction of "it is," the correct possessive form "its" should be used. This fix ensures grammatical correctness in the comment.

The checklist

• Associated issue
• A news fragment in the news/ directory to describe this fix with the extension .bugfix.rst, .feature.rst, .behavior.rst, .doc.rst. .vendor.rst. or .trivial.rst (this will appear in the release changelog). Use semantic line breaks and name the file after the issue number or the PR #.