Add reference to the MIGRATION.md file to the PR template checklist

[iangmaia]

What does it do?

1. Fixes a small typo on the Github PR template: describe your changes under the approprioate
2. Adds a Checklist item to remind folks to consider the implications of their changes when migrating from a previous major version.

Related PRs

• This PR is based on the initial MIGRATION.md discussions that started on Add MIGRATION.md migration guide #452

Checklist before requesting a review

• Run bundle exec rubocop to test for code style violations and recommendations
• Add Unit Tests (aka specs/*_spec.rb) if applicable
• Run bundle exec rspec to run the whole test suite and ensure all your tests pass
• Make sure you added an entry in the CHANGELOG.md file to describe your changes under the approprioate existing ### subsection of the existing ## Trunk section.

[AliSoftware]

LGTM! Ideally I'd love for us to configure Danger on that repo at some point to automate those checks, but until then, this is a good way to remind us about the MIGRATION.md file 👍