Update README to refer to correct plugin artifactId.

Description

Update README to refer to correct plugin artifactId of io.github.blackrock:protoc-maven-plugin.

Before submitting this pull request, please ensure that the following criteria have been met:

[X] All automated tests have passed successfully.
[X] All manual tests have passed successfully.
[X] Code has been reviewed by at least one other team member.
[X] Code has been properly documented and commented as needed.
[X] All new and existing code adheres to our project's coding standards.
[X] All dependencies have been added or removed from the project's README or other documentation as needed.
[X] Any relevant documentation or help files have been updated to reflect the changes made in this pull request.
[X] Any necessary database migrations have been run.
[X] Any relevant UI changes have been reviewed and approved by the UI/UX team.