docs(README): clarify that an existing open PR is managed (#3498)

* docs(README): clarify that an existing open PR is managed

* make it a little more concise

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Co-authored-by: Peter Evans <[email protected]>

Author: caugner

README.md

@@ -15,7 +15,8 @@ Create Pull Request action will:
    - tracked (modified) files
    - commits made during the workflow that have not been pushed
 2. Commit all changes to a new branch, or update an existing pull request branch.
-3. Create a pull request to merge the new branch into the base—the branch checked out in the workflow.
+3. Create or update a pull request to merge the branch into the base—the branch checked out in the workflow.
+   or update an existing open pull request for that branch.
 
 ## Documentation