Update CONTRIBUTING.md

What does this MR do?

Fixes spelling - changes Contributer to Contributor.

I'm setting up another repo, and wanted to hold this repo as an example for how to do it, but I noticed the spelling mistake and wanted to have it fixed first. 😅

I don't think this requires a changelog entry.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests updated/added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer