docs: update argument type for .contains to match impl

[danmana]

See discussion in DefinitelyTyped/DefinitelyTyped#56461

[codecov]

Codecov Report

Merging #2540 (7db06de) into master (c8c74a4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2540   +/-   ##
=======================================
  Coverage   96.31%   96.31%           
=======================================
  Files          49       49           
  Lines        4207     4207           
  Branches     1130     1130           
=======================================
  Hits         4052     4052           
  Misses        155      155           

Impacted Files	Coverage Δ
packages/enzyme/src/ReactWrapper.js	99.51% <ø> (ø)
packages/enzyme/src/ShallowWrapper.js	99.12% <ø> (ø)

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