Doc: spelling fixes

[WardBrian]

Submission Checklist

• Run unit tests
• Declare copyright holder and open-source license: see below

Summary

Typo fixes

Copyright and Licensing

Please list the copyright holder for the work you are submitting (this will be you or your assignee, such as a university or company):
Simons Foundation

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the following licenses:

• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[codecov-commenter]

Codecov Report

Merging #678 (17fc4ea) into develop (1361f35) will decrease coverage by 0.06%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #678      +/-   ##
===========================================
- Coverage    81.06%   81.00%   -0.06%     
===========================================
  Files           69       69              
  Lines        10677    10677              
===========================================
- Hits          8655     8649       -6     
- Misses        2022     2028       +6     

see 2 files with indirect coverage changes

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[mitzimorris]

lgtm! much needed