Fix docmentation mismatch

[smac89]

default for max-string-usages should be MAX_STRING_USAGES

I have made things!

Checklist

• I have double checked that there are no unrelated changes in this pull request (old patches, accidental config files, etc)
• I have created at least one test case for the changes I have made
• I have updated the documentation for the changes I have made
• I have added my changes to the CHANGELOG.md

Related issues

Refs #2392

[sobolevn]

Thanks! Can you please fix the changelog entry?

[codecov]

Codecov Report

Merging #2456 (f0470fb) into master (66a7c9a) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #2456   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          120       120           
  Lines         6422      6422           
  Branches      1447      1447           
=========================================
  Hits          6422      6422           

Impacted Files	Coverage Δ
wemake_python_styleguide/options/config.py	100.00% <ø> (ø)

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[sobolevn]

Thanks!