Set message to FALSE in knitr opts

[kelly-sovacool]

Checklist

(~Strikethrough~ any points that are not applicable.)

• [x] Write unit tests for any new functionality.
• ~[ ] Update roxygen comments & vignettes if there are any API changes.~
• [x] Update NEWS.md if this includes any user-facing changes.
• [x] The check workflow succeeds on your most recent commit.