Revise README for AP calculus BC support

[laurgao]

Updated the README to reflect support for all AP calculus BC integrals.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.96%. Comparing base (7672980) to head (9cd2b05).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #53      +/-   ##
==========================================
+ Coverage   92.92%   92.96%   +0.03%     
==========================================
  Files          14       14              
  Lines        2869     2871       +2     
==========================================
+ Hits         2666     2669       +3     
+ Misses        203      202       -1     

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