Fix budget mode selection in Internet Explorer 11

As mentioned in commit b7f6c4c43:

> [In Internet Explorer] the `flex: 1` property doesn't work so well
> when `flex-direction` is set to `column`. We're replacing it with
> `flex-grow: 1`. No need to set other `flex-basis` nor `flex-shrink`
> since in this case the default values will work just fine.

app/assets/stylesheets/admin/budgets/heading_mode.scss

@@ -56,7 +56,7 @@
   p {
     font-size: $small-font-size;
     font-style: italic;
-    flex: 1;
+    flex-grow: 1;
   }
 
   a {