Fix formatting for link to QualifiedName class in docs

[browniebroke]

Summary

While checking out the cods for the recently added FullyQualifiedNameProvider, I noticed that one link to a class wasn't properly handled by Sphinx:

I think the idea was to add an s at the end of the class name since we're talking about multiple of them, but I couldn't find how to make the s stick, so I've reworded a bit.

Alternative solutions I considered, with the reason why I didn't go for them:

• Specify a title with the "s" (:class:`QualifiedNames <QualifiedName>`): altering the class name might confuse people
• Add a space between the class name and the "s": formatting looks off
• Remove extra markup/formatting: seems like a step back vs having links to the class docs.

Test Plan

Check the docs output

[codecov-commenter]

Codecov Report

Merging #480 (54d3fe1) into master (3b21fa9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #480   +/-   ##
=======================================
  Coverage   94.68%   94.68%           
=======================================
  Files         236      236           
  Lines       23013    23013           
=======================================
  Hits        21790    21790           
  Misses       1223     1223           

Impacted Files	Coverage Δ
libcst/metadata/name_provider.py	100.00% <ø> (ø)

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[zsol]

Thanks for the fix!