fix grammar in test_collect_functools_partial docstr

[graingert]

• Target the master branch for bug fixes, documentation updates and trivial changes.
• Target the features branch for new features and removals/deprecations.
• Include documentation when adding new features.
• Include new tests or update existing tests when applicable.

Unless your change is trivial or a small documentation fix (e.g., a typo or reword of a small section) please:

• Create a new changelog file in the changelog folder, with a name like <ISSUE NUMBER>.<TYPE>.rst. See changelog/README.rst for details.
• Add yourself to AUTHORS in alphabetical order;

[codecov]

Codecov Report

Merging #5703 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5703      +/-   ##
==========================================
+ Coverage   96.16%   96.17%   +<.01%     
==========================================
  Files         117      117              
  Lines       25864    25864              
  Branches     2498     2498              
==========================================
+ Hits        24873    24875       +2     
+ Misses        690      689       -1     
+ Partials      301      300       -1

Impacted Files	Coverage Δ
testing/python/collect.py	99.36% <ø> (ø)	⬆️
src/_pytest/pastebin.py	91.22% <0%> (+3.5%)	⬆️

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[nicoddemus]

LGTM as is, thanks!