users: Update first boot step submit button text

'Box it up!' is confusing is and no longer relavent as there are more
steps after that step.

plinth/modules/users/templates/users_firstboot.html

@@ -42,7 +42,7 @@
       {{ form|bootstrap }}
 
       <input type="submit" class="btn btn-primary"
-             value="{% trans "Box it up!" %}"/>
+             value="{% trans "Create Account" %}"/>
     </form>
 
   </div>