Update solution.md

Same case: Don't know if this fixes the repetition but I can't check it till merged.

Author: maurodibert

1-js/05-data-types/03-string/1-ucfirst/solution.md

@@ -15,7 +15,7 @@ There are two variants here:
 
 Here's the 2nd variant:
 
-```js run
+```js run demo
 function ucFirst(str) {
   if (!str) return str;