localrepo: correct docstring of filectx()
The same reason as
b6c2543e1dd8. It can't be any changeset specifiers but
revision number.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
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273ce12ad8f1` will report some style violations on a very old commit).