doc: change "GitHub Repo" to "Code repository"

"GitHub Repo & Issue Tracker" changed to "Code repository and issue
tracker".

PR-URL: #33985
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Matheus Marchini <mat@mmarchini.me>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Myles Borins <myles.borins@gmail.com>

doc/api/index.md

@@ -63,4 +63,4 @@
 
 <div class="line"></div>
 
-* [GitHub Repo & Issue Tracker](https://github.com/nodejs/node)
+* [Code repository and issue tracker](https://github.com/nodejs/node)