Fix tense and sentence length

Changed past-perfect and present mixture to use just the present tense and added a comma.

Author: aalaap

security/form_login_setup.rst

@@ -171,9 +171,9 @@ form::
 
 Don't let this controller confuse you. As you'll see in a moment, when the
 user submits the form, the security system automatically handles the form
-submission for you. If the user had submitted an invalid username or password,
-this controller reads the form submission error from the security system so
-that it can be displayed back to the user.
+submission for you. If the user submits an invalid username or password,
+this controller reads the form submission error from the security system,
+so that it can be displayed back to the user.
 
 In other words, your job is to *display* the login form and any login errors
 that may have occurred, but the security system itself takes care of checking