update persistence spec link

gencapdefs.py

     CapDef(
         identifier="Persistence",
         name="draft/persistence",
-        url="https://gist.github.com/slingamn/e3645a0d0418b736b755746bfd65f2a6",
+        url="https://github.com/ircv3/ircv3-specifications/pull/503",
         standard="proposed IRCv3",
     ),
 ]

irc/caps/defs.go

 	Multiline Capability = iota
 
 	// Persistence is the proposed IRCv3 capability named "draft/persistence":
-	// https://gist.github.com/slingamn/e3645a0d0418b736b755746bfd65f2a6
+	// https://github.com/ircv3/ircv3-specifications/pull/503
 	Persistence Capability = iota
 
 	// ReadMarker is the draft IRCv3 capability named "draft/read-marker":