DOC: Fix typos and standardize spelling of "GitHub"  and "macOS"
## Proposed changes

1. Fix some typos. They were found with:

   ```shell
   brew install aspell
   
   find . -type f -exec cat '{}' \; |
     aspell -a |
     grep -v '*' |
     sed '/^$/d' |
     cut -d ' ' -f 2 |
     sort -u
   ```

   The output was visually inspected to find the typos.

3. Standardize spelling of "GitHub". The official spelling of "GitHub" can be seen in the title of page <https://github.com/>.
4. Standardize spelling of "macOS". The Apple operating system was called "Mac OS X" in versions 10.0-10.7, then "OS X" in versions 10.8-10.11, and finally "macOS" since version 10.12, released in 2016 (<https://en.wikipedia.org/wiki/MacOS#Release_history>).

## Checklist

- [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

thx @gcbeltramini 