Update example pre-commit install instructions to latest hooks version.

[mattseymour]

The quick start instructions for pre-commit point to version 2.3.0 of pre-commit-hooks (2019). We should update the quick start instructions to use the latest version of hooks (v6.0.0 as of today).

[mattseymour]

See #1097 for PR

[asottile]

search next time!

[mattseymour]

? sorry what do you mean.

If you are referring to this #806 then the fact multiple people are falling foul to this means its probably not clear nor explained very well in the docs.

[asottile]

just because lots of people make the same mistake of not searching before opening a duplicate issue or pr does not mean you're absolved