watch.css: Fix from d2c -> d2 rename

cc @alixander this has been broken for so long lol
From when the code was in the monorepo.

cmd/d2/static/watch.css

@@ -1,4 +1,4 @@
-#d2c-err {
+#d2-err {
   /* This style was copied from Chrome's svg parser error style. */
   white-space: pre-wrap;
   border: 2px solid #c77;