Update placeholder text.

app/templates/sessions/new.mustache

@@ -1,6 +1,6 @@
 <form id="login-form" action="{{ sessions_path }}" method="post">
   <input type="hidden" name="authenticity_token" value="{{ form_authenticity_token }}"/>
-  <input type="text" name="username" placeholder="User name"/>
+  <input type="text" name="username" placeholder="Username"/>
   <input type="password" name="password" placeholder="Password"/>
   <input type="submit" value="Sign in"/>
 </form>