[docs] [issue22603] Fix a typo in the contextlib docs

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 ---------- nosy: +python-dev _______________________________________ Python tracker <report@bugs.python.org> <http://bugs.python.org/issue22603> _______________________________________