[pre-commit] Fix yamllint errors in .pre-commit-config.yaml

I ran yamllint [1] on .pre-commit-config.yaml and ran into a few errors:
- missing document start marker
- indentation errors
- white space error

Fix these.

With git show -w, the only change is adding the missing document start marker.

[1] https://github.com/adrienverge/yamllint