Fix whitespace in example

doc/src/manual/cowboy.set_env.asciidoc

@@ -64,7 +64,7 @@ not exist.
 Dispatch = cowboy_router:compile([
     {'_', [
         {"/", toppage_h, []},
-		{"/ws", websocket_h, []}
+        {"/ws", websocket_h, []}
     ]}
 ]),