fix syntax of README file

[aaarendt]

I accidentally included Sphinx specific interpreted text roles in the readme file (I copied them out of the Jupyterbook docs, which does interpret Sphinx code correctly). That problem is causing our current build error. This PR fixes that.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.52%. Comparing base (bf33a5f) to head (9380c9a).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #211   +/-   ##
=======================================
  Coverage   59.52%   59.52%           
=======================================
  Files          23       23           
  Lines        1218     1218           
=======================================
  Hits          725      725           
  Misses        493      493           

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