Switch to \mathcal{R}(t) in docs/index.md

[dylanhmorris]

Consistent with changes in #681

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.46%. Comparing base (fa6aa05) to head (0b3d1d7).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #694   +/-   ##
=======================================
  Coverage   97.46%   97.46%           
=======================================
  Files          51       51           
  Lines        1540     1540           
=======================================
  Hits         1501     1501           
  Misses         39       39           

Flag	Coverage Δ
unittests	97.46% <ø> (ø)

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[github-actions]

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