Fix 4.1 Installation Doc wording

Created by: slottermoser

Just some wording in the installation doc for GitLab 4.1 that needed to be fixed.

app/controllers/profiles_controller.rb

@@ -51,7 +51,9 @@ class ProfilesController < ApplicationController
   end
   def update_username
-    @user.update_attributes(username: params[:user][:username])
+    if @user.can_change_username?
+      @user.update_attributes(username: params[:user][:username])
+    end
     respond_to do |format|
       format.js