DOC: Remove duplicate 'in' from contributing.rst (#18040)
- [x] closes #18040 (already closed)
- [ ] Tests added / passed (N/A)
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry (N/A)


Removes a duplicate "in" from the second sentence of the last paragraph of the "About the _pandas_ documentation" section of the "Contributing to Pandas" docs.