Remove stray double-quote

[lurch]

The help-string for --error-format is triple-quoted, so remove unnecessary quote character

[codecov]

Codecov Report

Merging #926 (f4a8619) into main (ee5d31a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #926   +/-   ##
=======================================
  Coverage   97.02%   97.02%           
=======================================
  Files          20       20           
  Lines        3295     3295           
  Branches      458      458           
=======================================
  Hits         3197     3197           
  Misses         77       77           
  Partials       21       21           

Impacted Files	Coverage Δ
jsonschema/cli.py	97.01% <ø> (ø)

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[Julian]

Thanks!