docs(eslint-plugin): [prefer-readonly-parameter-types] fix invalid example

[bradzacher]

Fixes #1789

[typescript-eslint]

Thanks for the PR, @bradzacher!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[codecov]

Codecov Report

Merging #1791 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1791   +/-   ##
=======================================
  Coverage   95.02%   95.02%           
=======================================
  Files         159      159           
  Lines        7112     7112           
  Branches     2033     2033           
=======================================
  Hits         6758     6758           
  Misses        154      154           
  Partials      200      200           

Flag	Coverage Δ
#unittest	95.02% <ø> (ø)