Fix typo in docs

[pablogamboa]

What do these changes do?

Are there changes in behavior for the user?

Related issue number

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

[codecov]

Codecov Report

Merging #541 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #541   +/-   ##
=======================================
  Coverage   98.38%   98.38%           
=======================================
  Files          31       31           
  Lines        5017     5017           
=======================================
  Hits         4936     4936           
  Misses         81       81

Impacted Files	Coverage Δ
aiokafka/conn.py	99.11% <0%> (-0.59%)	⬇️
aiokafka/consumer/group_coordinator.py	99.16% <0%> (+0.27%)	⬆️

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[tvoinarovskyi]

Thanks!