Correct rst syntax in installation section

[jaap3]

Description of the Change

I noticed that the installation section in the README was using single backticks (`) instead of double backticks (``) to mark up some code. This works in Markdown, but doesn't really work in reStructuredText. While I was there I also tagged urls.py.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.42%. Comparing base (560f84d) to head (f56cf9e).
Report is 53 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1434      +/-   ##
==========================================
- Coverage   97.56%   97.42%   -0.14%     
==========================================
  Files          32       32              
  Lines        2132     2096      -36     
==========================================
- Hits         2080     2042      -38     
- Misses         52       54       +2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.