docs: update expect.anything() example

[alopezsanchez]

Fixes: #4916

Summary
Update expect.anything() sample code. Calling [1].map(mock); fails because map adds two extra parameters to the mock call (index and array).

Test plan
expect-anything.test.js

test('map calls its argument with a non-null argument', () => {
  const mock = jest.fn();
  [1].map(x => mock(x));
  expect(mock).toBeCalledWith(expect.anything());
});

> jest "test/expect-anything.test.js"

 PASS  test/expect-anything.test.js
  ✓ map calls its argument with a non-null argument (5ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        0.592s, estimated 1s
Ran all test suites matching /test\/expect-anything.test.js/i.

[codecov-io]

Codecov Report

Merging #5007 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5007   +/-   ##
=======================================
  Coverage   60.31%   60.31%           
=======================================
  Files         198      198           
  Lines        6662     6662           
  Branches        3        3           
=======================================
  Hits         4018     4018           
  Misses       2644     2644

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[SimenB]

Thanks!

[cpojer]

Thanks for fixing up our docs!

[github-actions]

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