Fix various spelling errors

[ottok]

Most fixes are in comments and have no effect on functionality.
Some fixes are also in variable names but they should be safe to change,
as the change is consistent in all occurrences of the variable.

I intend to fix submit this on mailing list following https://wiki.postgresql.org/wiki/Submitting_a_Patch but decided to open a PR here too to benefit from the CI system PostgreSQL has on Pull Requests.

[dpage]

Thanks for your Pull Request! 😄 This repo on GitHub is just a mirror of our real git repositories though, and can't really handle PRs. 😦 Hopefully you can redo the PR, and direct it to the git.postgresql.org repos? We have a developer guide, if that helps: https://wiki.postgresql.org/wiki/So,_you_want_to_be_a_developer%3F. If this was a PR for pgAdmin, please visit https://www.pgadmin.org/docs/pgadmin4/dev/submitting_patches.html, or if it was a patch for pgweb, please send it to pgsql-www@lists.postgresql.org.

[anarazel]

@ottok You can enable the CI on your own fork as well. https://github.com/postgres/postgres/blob/master/src/tools/ci/README

[ottok]

This was committed in 6bdf1a1 (although without using proper git --author, so Github and other trackers don't show me as author, but main thing is that is in now).