Fix inconsistent formatting in CHANGELOG.md (#1031)
Fix inconsistent formatting in CHANGELOG.md (#1031)

- Switches releases to always using H2 (`##`) rather than a mixture of
  H1s (meaning multiple H1s in the same document) and H2s.
- Always refers to the versions as `vNNN` rather than sometimes without
  the `v` prefix.
- Other formatting and typo fixes.

@W-7905079@
Scalingo/python-buildpack,heroku/heroku-buildpack-python,heroku/heroku-buildpack-python,AltSchool/heroku-buildpack-python,AltSchool/heroku-buildpack-python,Scalingo/python-buildpack,skilljar/heroku-buildpack-python