Update 3.9.0a2.rst - correct small typos (GH-17665)

Small typo/formatting corrections.
`whethen` -> `whether` 
`exaustion' -> `exhaustion`
Assorted appending periods `.` and slight reformattings to place `Path contributed by` on the same line as description, matching the majority of document.

NB Some of these might need to be backported, as I saw the first error in the [changelog for 3.8.1](https://docs.python.org/3.8/whatsnew/changelog.html#python-3-8-1)