Spelling fix. no bug. rs=dmose

calendar/locales/en-US/chrome/calendar/calendar.dtd

@@ -56,7 +56,7 @@
 <!ENTITY alarm.start.warning     "This alarm requires a starting date" >
 <!ENTITY alarm.due.warning     "This alarm requires a due date" >
 
-<!ENTITY repeat.units.occurence.both         "occurence(s)" >
+<!ENTITY repeat.units.occurence.both         "occurrence(s)" >
 
 <!ENTITY repeat.units.days.both         "Day(s)" >
 <!ENTITY repeat.units.weeks.both        "Week(s)" >