Update Plugin-API.md

[eyyyyyyy3]

Fixed the "RCP" typo to "RPC"

What it does

How to test

Follow-ups

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[JonasHelming]

Hi @eyyyyyyy3 ,

Thank you for the contribution and the fix! In order to merge this, we would need you to sign the Eclipse Contributor Agreement (https://api.eclipse.org/git/eca/status/gh/eclipse-theia/theia/14254). With this, you agree to publish your contribution open source.
This might look like a big overhead for a simple typo fix, but as Theia is adopted in a commercial context, we need to follow this process to ensure IP compliance. The ECA will only need to be signed once and is then valid for all future contributions.

[eyyyyyyy3]

Done