Fix branch name discrepency in pr-lint.yml

[adrienntindall]

I forgot that the main branch is named differently in pokeheartgold, so the linter wasn't triggering before

PR checklist

• This comment contains a description of changes (with reason).
• The ROM compiles to matching locally (make compare_heartgold && make compare_soulsilver).
• All C code is correctly formatted (git clang-format).
• This pull request is labeled according to the kind of work it represents.
• New or updated code labels follow the style guide
• This work adheres to the AI policy.
• The author has joined the pret discord server and sent a message in #pokeheartgold to verify that they are human