Corrected grammar mistake in documentation

[shobute]

There was an unneeded space before a closing parenthesis in the unittest.mock documentation.

[mention-bot]

@shobute, thanks for your PR! By analyzing the history of the files in this pull request, we identified @mfoord, @birkenfeld and @serhiy-storchaka to be potential reviewers.

[the-knights-who-say-ni]

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[serhiy-storchaka]

Thank you for your patch @shobute. Do you mind to backport this change to the 3.6 and 3.5 branches?