sponsors: update coderabbit URL

Author: justinmk

index.html

@@ -81,7 +81,7 @@ <h3>Drop-in Vim</h3>
       <h2 id="sponsors-header">Sponsors</h2>
       <div id="sponsors" class="sponsors-main">
         <div class="first-level-sponsor">
-          <a href="https://coderabbit.ai/">
+          <a href="https://coderabbit.ai/?utm_source=neovim.io">
             <picture title="Visit coderabbit.ai to learn more." alt="Visit coderabbit.ai to learn more.">
               <!-- light mode: -->
               <source srcset="images/sponsors/coderabbit-logo.svg" media="(prefers-color-scheme: light)"/>