HTML up to specs (#804)

</img> doesn't exist so i removed it
also i added a / at the end of <img> because <br /> has it

templates/404.html

@@ -3,6 +3,6 @@
     <div>
         <h1>{{call $.T "404_not_found"}}</h1>
         <br />
-        <img class="center-block center-image" src="/img/404.svg"></img>
+        <img class="center-block center-image" src="/img/404.svg" />
     </div>
 {{end}}