docs: Add terminal output ss

Author: adi-ray

src/content/contributor-docs/en/contributing_to_the_p5js_reference.mdx

@@ -401,6 +401,9 @@ npm run custom:dev https://github.com/yourUsername/p5.js.git#yourBranch
 
 This will build the reference from your branch and start a development preview of the website. A URL will be logged in the console that you can go to in your browser to test out your changes.
 
+If everything is working correctly, your terminal output should look similar to this:
+![Terminal output showing successful npm run custom:dev execution](src/content/contributor-docs/images/custom-dev-terminal-output.png)
+
 * When you're done, you can run this command to reset your changes:
 
 ```