[Date] fix duplicate word

Author: moritz

lib/Date.pod

@@ -9,7 +9,7 @@ A C<Date> is an immutable object identifying a day in the Gregorian calendar.
 Date objects support addition and subtraction of integers, where an
 integer is interpreted as the number of days. You can compare Date objects
 with the numeric comparison operators C<< ==, <, <=, >, >=, != >>.
-Their stringification in C<YYYY-MM-DD> format means that that comparing them
+Their stringification in C<YYYY-MM-DD> format means that comparing them
 with the string operators C<eq, lt, le> etc. also gives the right result.
 
 C<Date.today> creates