fixed a typo (flat -> flag)

docs/documentation/customize/concepts.html

@@ -37,7 +37,7 @@ breadcrumb:
   </ul>
 
   <p>
-    Since these variables carry the <code>!default</code> flat, they can be assigned a <strong>new value</strong> either before or after having been imported.
+    Since these variables carry the <code>!default</code> flag, they can be assigned a <strong>new value</strong> either before or after having been imported.
   </p>
 </div>