Fix typo in _pkgdown.yml

[jgabry]

Summary

closes #404, thanks to @ctholley

Fixes typo in _pkgdown.yml

Copyright and Licensing

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/)

[jgabry]

Failures are unrelated to the PR