Remove extraneous R-squared term from formula for radiated power within an angular cone in tutorial

[oskooi]

Fixes the formula for the radiated power within an angular cone in Tutorial/Nonaxisymmetric Dipole Sources by removing an extraneous $R^2$ term. This formula appears in the third list item:

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.84%. Comparing base (f29a8c7) to head (e126b6c).
⚠️ Report is 91 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3117      +/-   ##
==========================================
+ Coverage   73.81%   73.84%   +0.02%     
==========================================
  Files          18       18              
  Lines        5423     5452      +29     
==========================================
+ Hits         4003     4026      +23     
- Misses       1420     1426       +6     

see 1 file with indirect coverage changes

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