Fix typo in README

[jgabry]

Submission Checklist

• Run unit tests
• Declare copyright holder and agree to license (see below)

Summary

Fix a minor typo in the README

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):
Columbia University

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)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (37b8cd3) 88.28% compared to head (e2d04b0) 86.65%.

❗ Current head e2d04b0 differs from pull request most recent head 63a29c5. Consider uploading reports for the commit 63a29c5 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #911      +/-   ##
==========================================
- Coverage   88.28%   86.65%   -1.64%     
==========================================
  Files          12       12              
  Lines        4534     4534              
==========================================
- Hits         4003     3929      -74     
- Misses        531      605      +74     

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