Fix incorrect sample filename

Created by: selwyntcy

Created by: totten

Thanks, @selwyntcy . I haven't read this page in a long time and don't immediately recognize the problem being fixed. Trying to read+summarize, it seems like:

• Background: There are supposed to be two files -- for a class and an entry-point.
• Before the patch: The short names of the two files were the same (i.e. UCMPaymentCollectionIPN.php, although maybe the folders could be different) -- and this was confusing.
• After the patch: With the patch, the names are clearly different (UCMPaymentCollectionNotify.php vs UCMPaymentCollectionIPN.php).

Have I understood correctly?

Created by: selwyntcy

Hm. Both line 302 and line 308 mention that there should be a file named UCMPaymentCollectionIPN.php. However, the filename above such sample code is incorrectly written as UCMPaymentCollectionNotify.php, the same as that of the previous sample code. I guess it was a copy & paste error.

@totten the whole page could do with an update. But this PR is an improvement and should be merged.

Created by: totten

Thanks, @mattwire