Update Swift snippet in iOS guide for user doc (#741)

* Updated Swift usage

Updated Swift usage based on
parse-community/Parse-SDK-iOS-OSX#1311

* Update Swift login snippet to work with current version of Swift

Update to change

1.  NSError to Error when logging in
2. use updated function name

* Updated currentUser() to current()

Updated currentUser() to current() when checking for currentuser

Co-authored-by: Jayson Ng <[email protected]>

Author: jaysonng

_includes/ios/users.md

@@ -87,8 +87,8 @@ Of course, after you allow users to sign up, you need to let them log in to thei
 }];
 ```
 ```swift
-PFUser.logInWithUsernameInBackground("myname", password:"mypass") {
-  (user: PFUser?, error: NSError?) -> Void in
+PFUser.logInWithUsername(inBackground:"myname", password:"mypass") {
+  (user: PFUser?, error: Error?) -> Void in
   if user != nil {
     // Do stuff after successful login.
   } else {
@@ -125,7 +125,7 @@ if (currentUser) {
 }
 ```
 ```swift
-var currentUser = PFUser.currentUser()
+var currentUser = PFUser.current()
 if currentUser != nil {
   // Do stuff with the user
 } else {