Docs: complete cut-off sentence in “Preparing a pull request” section

[sdass1918]

Fixes #3596

This PR fixes a cut-off sentence in contributor_docs/preparing_a_pull_request.md under the “Pull Request Templates” section.

Changes: Completed the missing text explaining the verification steps contributors should check before submitting a PR.

I have verified that this pull request:

• [x] has no linting errors (npm run lint)
• [x] has no test errors (npm run test)
• [x] is from a uniquely-named feature branch and is up to date with the develop branch.
• [x] is descriptively named and links to an issue number, i.e. Fixes #123
• [x] meets the standards outlined in the accessibility guidelines