Update guide to tell user where to get "original" Portfile for patching

[jfcaron3]

If you read: ​https://guide.macports.org/#development.patches.portfile

It tells you how to make a copy and generate patch for a Portfile, but it does not tell you where to find the original Portfile. Looking on my system with find /opt/local grep|portname|grep Portfile, I find two candidates, one somewhere in /opt/localvar/macports/sources/rsync.macports.org/release/tarballs/ports and another somewhere in /opt/localvar/macports/registry/portfiles/ and they appear slightly different.

Even if this is an idiosyncrasy in my system, the guide section should include the default location of the Portfile to be used to generate patches.