Fix typo

[cclauss]

What do these changes do?

https://github.com/aio-libs/frozenlist/pull/307/files

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
• If you provide code modifications, please add yourself to CONTRIBUTORS.txt
  • The format is <Name> <Surname>.
  • Please keep the list in alphabetical order, the file is sorted by name.
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> for example (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 #307 (adafec0) into master (f92e9c6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #307   +/-   ##
=======================================
  Coverage   98.36%   98.36%           
=======================================
  Files           1        1           
  Lines          61       61           
  Branches        7        7           
=======================================
  Hits           60       60           
  Misses          1        1           

Flag	Coverage Δ
unit	96.72% <ø> (ø)

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