Update Commit example documentation to mention index.write after committing

The documentation mentions, in one place, at least, how to commit a newly written index (using index.write_tree(repo)), but fails to mention that index.write should be called after the commit has been performed. This is a bit unintuitive to me, so it'd be nice if the API could be improved, but barring that, at least add that one line to the documentation (index.write).

This issue seems somewhat common -- it's come up at least twice: on GitHub as well as on StackOverflow. On the SO question, Carlos does say That example does not emulate the git-commit command, it just creates a new commit., but since some people (at least me and two others) expect it to work this way, it seems like it'd be worth calling out, at least.

Open the linked “Writing to a repository” documentation section and read the existing commit example. Update that example to mention index.write after the commit, while preserving the existing index.write_tree(repo) guidance. Done means the documented sequence clearly explains how to persist the updated index.