Fix indentation in readme's pre-commit config

[jvacek]

just a couple spaces

[abravalheri]

Thank you very much!

[glensc]

please make release with this.

❯ git commit -am 'Add validate-pyproject pre-commit hook'
An error has occurred: InvalidConfigError: 
==> File .pre-commit-config.yaml
=====> while parsing a block collection
  in "<unicode string>", line 33, column 7
did not find expected '-' indicator
  in "<unicode string>", line 35, column 7
Check the log at /Users/glen/.cache/pre-commit/pre-commit.log

because I took sample from pypi and got the same error, which I was about to submit as fix here

• https://pypi.org/project/validate-pyproject/0.19/