Fix incorrect m.OneOf() docs

[Zac-HD]

Closes #345, which I'd just rediscovered and written a nice reproducer for 😅

[codecov-commenter]

Codecov Report

Merging #529 (ad3bb68) into main (13485d3) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #529      +/-   ##
==========================================
- Coverage   94.77%   94.77%   -0.01%     
==========================================
  Files         236      235       -1     
  Lines       23143    23141       -2     
==========================================
- Hits        21934    21932       -2     
  Misses       1209     1209              

Impacted Files	Coverage Δ
libcst/matchers/_matcher_base.py	90.97% <ø> (ø)
libcst/_version.py

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