Use immutable ref in pre-commit docs

[hukkin]

Use immutable ref instead of a branch name in pre-commit usage instructions.

Here's the pre-commit docs and rationale on this: https://pre-commit.com/#using-the-latest-version-for-a-repository

This PR uses one of the two documentation style suggestions by the pre-commit author psf/black#420 (comment)

Note that a version that includes the .pre-commit-hooks.yml file needs to be tagged before this works. I made the documented example version 0.2.0 but perhaps it should be 0.1.6? Which one is expected to be released next?

[codecov]

Codecov Report

Merging #230 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #230   +/-   ##
=======================================
  Coverage   97.59%   97.59%           
=======================================
  Files          22       22           
  Lines         457      457           
  Branches       64       64           
=======================================
  Hits          446      446           
  Misses          8        8           
  Partials        3        3           

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[sobolevn]

Thanks a lot! Very good practice.