[issue22603] Fix a typo in the contextlib docs

[Roundup Robot]

Roundup Robot added the comment:

New changeset b81443d9c64a by Terry Jan Reedy in branch '3.4':
Issue 22603: add missing 'self'.  Patch by Francisco Fernández Castaño.
https://hg.python.org/cpython/rev/b81443d9c64a

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nosy: +python-dev

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