Fix spelling and grammar in some comments

[stroxler]

I found this when I was looking at what tooling I could adapt to understanding dmypy suggest outputs (which look a lot like type comments!)

[codecov-commenter]

Codecov Report

Patch and project coverage have no change.

Comparison is base (f936db2) 94.88% compared to head (f8e4dcf) 94.88%.

❗ Current head f8e4dcf differs from pull request most recent head 734ede6. Consider uploading reports for the commit 734ede6 to get more accurate results

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #908   +/-   ##
=======================================
  Coverage   94.88%   94.88%           
=======================================
  Files         254      254           
  Lines       25977    25977           
=======================================
  Hits        24648    24648           
  Misses       1329     1329           

Impacted Files	Coverage Δ
libcst/codemod/commands/convert_type_comments.py	94.98% <ø> (ø)

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