doc: add missing semicolon

PR-URL: #7915
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/assert.md

@@ -258,7 +258,7 @@ const obj3 = {
   a : {
     b : 1
   }
-}
+};
 const obj4 = Object.create(obj1);
 
 assert.notDeepEqual(obj1, obj1);