Update Healthchecks instructions (#738)
Update Healthchecks instructions (#738)

- remove the note about Django 2.2 being the only supported version
  (3.2 seems to work too)
- change the install instructions to install from PyPI
- fix Markdown formatting for python code blocks

Co-authored-by: Vikash Singh <1319130075df6bda0c48aec63efc40780c306d41@users.noreply.github.com>
googleapis/python-spanner-django,googleapis/python-spanner-django