Update index.html (#328)

Update invalid links from 'master' outdated branch to 'main' branch for VSCode CSS/HTML.

Author: ahndmal

index.html

@@ -757,7 +757,7 @@ <h4>Language servers</h4>
             <a href="https://www.microsoft.com/">Microsoft</a>
           </td>
           <td class="repo">
-            <a href="https://github.com/Microsoft/vscode/tree/master/extensions/css">github.com/Microsoft/vscode/tree/master/extensions/css</a>
+            <a href="https://github.com/Microsoft/vscode/tree/main/extensions/css">github.com/Microsoft/vscode/tree/main/extensions/css</a>
           </td>
           <td class="success">
             <span class="glyphicon glyphicon-ok" aria-hidden="true"></span>
@@ -1674,7 +1674,7 @@ <h4>Language servers</h4>
             <a href="https://www.microsoft.com/">Microsoft</a>
           </td>
           <td class="repo">
-            <a href="https://github.com/Microsoft/vscode/tree/master/extensions/html">github.com/Microsoft/vscode/tree/master/extensions/html</a>
+            <a href="https://github.com/Microsoft/vscode/tree/main/extensions/html">github.com/Microsoft/vscode/tree/main/extensions/html</a>
           </td>
           <td class="success">
             <span class="glyphicon glyphicon-ok" aria-hidden="true"></span>