Fix bad crossref in RDoc for ActiveSupport::CoreExtensions::Date::Calculations

activesupport/lib/active_support/core_ext/date/calculations.rb

 module ActiveSupport #:nodoc:
   module CoreExtensions #:nodoc:
     module Date #:nodoc:
-      # Enables the use of time calculations within Time itself
+      # Enables the use of time calculations within Date itself
       module Calculations
         def self.included(base) #:nodoc:
           base.extend ClassMethods