Fix snippet page's header class (#3717, #3703)

wagtail/wagtailsnippets/templates/wagtailsnippets/snippets/type_index.html

@@ -17,7 +17,7 @@
 
     <header class="nice-padding">
         <div class="row row-flush">
-            <div class="left col9 header-snippet">
+            <div class="left col9 header-title">
                 <h1 class="icon icon-snippet">
                 {% blocktrans with snippet_type_name_plural=model_opts.verbose_name_plural|capfirst %}Snippets <span>{{ snippet_type_name_plural }}</span>{% endblocktrans %}</h1>