Fix "Good Beginner Bugs" link

Seems like the label was changed and the link needs to be updated

README.md

@@ -14,7 +14,7 @@ get involved, visit:
 + [Issue Reporting Guide](https://github.com/mozilla/pdf.js/blob/master/.github/CONTRIBUTING.md)
 + [Code Contribution Guide](https://github.com/mozilla/pdf.js/wiki/Contributing)
 + [Frequently Asked Questions](https://github.com/mozilla/pdf.js/wiki/Frequently-Asked-Questions)
-+ [Good Beginner Bugs](https://github.com/mozilla/pdf.js/issues?direction=desc&labels=5-good-beginner-bug&page=1&sort=created&state=open)
++ [Good Beginner Bugs](https://github.com/mozilla/pdf.js/issues?direction=desc&labels=good-beginner-bug&page=1&sort=created&state=open)
 + [Projects](https://github.com/mozilla/pdf.js/projects)
 
 Feel free to stop by our [Matrix room](https://chat.mozilla.org/#/room/#pdfjs:mozilla.org) for questions or guidance.