Typo in apply! docs

[kreutzheidephil]

Typo in apply! doc when explaining which part of the stiffness matrix is used during mutation of the rhs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.31%. Comparing base (9134b54) to head (a6cf4b2).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1313   +/-   ##
=======================================
  Coverage   94.31%   94.31%           
=======================================
  Files          40       40           
  Lines        6768     6768           
=======================================
  Hits         6383     6383           
  Misses        385      385           

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