Fix broken links in the PR template

[obnoxxx]

Links were broken due to using a nonexistent devel branch. Fix this by using maininstead.

Describe what this PR does

This fixes broken links in the PR template.

Is there anything that requires special attention

no

Do you have any questions?

no

Is the change backward compatible?

yes

Are there concerns around backward compatibility?

no

Provide any external context for the change, if any.

none

Related issues

Fixes: #168

Future concerns

none Checklist:

• [ ] Commit Message Formatting: Commit titles and messages follow guidelines in the developer guide.
• [ ] Reviewed the developer guide on Submitting a Pull Request
• [ ] Pending release notes updated with breaking and/or notable changes for the next major release.
• [ ] Documentation has been updated, if necessary.
• [ ] Unit tests have been added, if necessary.
• [ ] Integration tests have been added, if necessary.