Miscellaneous comment and doc corrections.

[WarrenWeckesser]

Low priority, but as long as I'm poking around in these files, I might as well fix the typos as I find them.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.83%. Comparing base (7e5e920) to head (2fda707).
Report is 5 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1239   +/-   ##
========================================
  Coverage    93.83%   93.83%           
========================================
  Files          657      657           
  Lines        55244    55244           
========================================
  Hits         51840    51840           
  Misses        3404     3404           

Files with missing lines	Coverage Δ
include/boost/math/distributions/skew_normal.hpp	84.72% <ø> (ø)
test/test_holtsmark.cpp	100.00% <ø> (ø)
test/test_landau.cpp	100.00% <ø> (ø)
test/test_mapairy.cpp	100.00% <ø> (ø)
test/test_saspoint5.cpp	100.00% <ø> (ø)

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[mborland]

Thanks you!

[ckormanyos]

Thank you Warren