Fix the link which was not supporting in README.md (#8229)

* Fix the link which was not supporting in README.md

I have updated the link to the CSS of matrix-react-sdk.

* Update README.md

README.md

@@ -67,7 +67,7 @@ practices that anyone working with the SDK needs to be aware of and uphold:
 
   * The view's CSS file MUST have the same name (e.g. view/rooms/MessageTile.css).
     CSS for matrix-react-sdk currently resides in
-    https://github.com/vector-im/element-web/tree/master/src/skins/vector/css/matrix-react-sdk.
+    https://github.com/matrix-org/matrix-react-sdk/tree/master/res/css.
 
   * Per-view CSS is optional - it could choose to inherit all its styling from
     the context of the rest of the app, although this is unusual for any but