fix case in flex example

Author: bissectra

flow/block-inline/flex.html

@@ -92,7 +92,7 @@
       </textarea>
   <textarea id="code" class="playable-html">
 <div class="container">
-    <div>FLex Item</div>
+    <div>Flex Item</div>
     <div>Flex Item</div>
     <div>
         <div>Children</div>