Update CONTRIBUTING.md (#6288)

Description
This PR fixes a minor typo in the CONTRIBUTING.md file to improve
readability and clarity. The updated instructions now use consistent
phrasing when guiding contributors on creating a pull request after
fixing typos.

Changes Made
Updated the sentence:
Old:
"To start, click the page icon... Once you've fixed the typo, commit
your changes to a new branch and create a pull request.
New:
"To begin, click the page icon... branch in your forked repository and
create a pull request.
Reason for the Change
This change improves the flow and consistency of the instructions,
making them clearer for first-time contributors.

Checklist
 I have read the contribution guidelines.
 My changes follow the project's style and formatting.

Linked Issue
This PR does not resolve any existing issues but enhances the
documentation for clarity.

Additional Notes
This is my first time contributing and Thank you for maintaining such an
inclusive project! Let me know if there's anything you'd like me to
adjust or improve further.

Author: bingolizo

CONTRIBUTING.md

@@ -4,10 +4,10 @@ Thanks for thinking about helping with [dart.dev][www]!
 You can contribute in a few ways.
 
 * **Fix typos.** The GitHub UI makes it easy to contribute small fixes, and
-  you'll get credit for your contribution! To start, click the **page icon**
+  you'll get credit for your contribution! To begin, click the **page icon**
   at the upper right of the page. Then click the **pencil icon** to start
   editing the file. Once you've fixed the typo, commit your changes to a new
-  branch and create a **pull request.**
+  branch in your forked repository and create a **pull request.**
 
   Once we've reviewed and approved your change, we'll merge it. Normally, we'll
   review your fix within one working day, and your fix will appear online less