Bad doco link for 1.3.1

[grply69]

Hello

the documentation pointed by 'how to upgrade?' on the download page (https://www.mantisbt.org/download.php)
is currently pointing to the *1.2* documentation.
This matters because Mantis 1.2 needs PHP 5.2, while 1.3 needs 5.3 minimum.
5.2 != 5.3 in significant ways: on Windows, there is no more isapi.dll, you have to change to fastcgi, but you are learning
this only after beginning to install.
IMO the 1.3 upgrade doco should really stress this PHP version difference; it's not very nice if you have planned for a quick install..

[atrol]

I fixed it.
Thanks for the hint.