Fix comment syntax error in CSS

Introduced in d19991c7.  //-style comments aren't supported in CSS.

This was causing update-prod-static to fail with a stack overflow.

(imported from commit 4cb11a7025e8fef32c852d691fccf18a5ad3b955)

zephyr/static/styles/zephyr.css

@@ -819,7 +819,7 @@ table.floating_recipient {
 }
 
 #stream-creation .modal-body {
-    // Setting max-height to a percentage doesn't work.
+    /* Setting max-height to a percentage doesn't work. */
     max-height: 400px;
     overflow-y: scroll;
 }