Add Proleptic Gregorian calendar link

Author: derickr

entities/global.ent

@@ -377,6 +377,7 @@
 <!ENTITY url.phpdbg.docs "https://phpdbg.room11.org/">
 <!ENTITY url.phpeditorlist "http://en.wikipedia.org/wiki/List_of_PHP_editors">
 <!ENTITY url.powerarchiver "http://www.powerarchiver.com/">
+<!ENTITY url.proleptic-gregorian-calendar "https://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar">
 <!ENTITY url.pslib "http://pslib.sourceforge.net/">
 <!ENTITY url.pspell "http://aspell.net/">
 <!ENTITY url.qdbm "http://fallabs.com/qdbm/index.html">