Replace master references

Author: oschwald

README.dev.md

@@ -96,7 +96,7 @@ Add this to ~/.gnupg/gpg-agent.conf:
 ## Steps
 - Ensure you can run `mvn test` and `mvn package` successfully. Run
   `mvn clean` after.
-- Ensure you have a clean checkout of `master` and that the subdirectory
+- Ensure you have a clean checkout of `main` and that the subdirectory
   `.gh-pages` either does not exist or is a clean checkout.
   - We'll be generating commits.
 - Review open issues and PRs to see if any can easily be fixed, closed, or