fix typo in `DateAndTime::Calculations#all_week` doc [ci skip]

`Date.week_start` does not exist. `Date.beginning_of_week` seems to be correct.
Ref: #5339

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

@@ -300,7 +300,7 @@ def all_day
     end
 
     # Returns a Range representing the whole week of the current date/time.
-    # Week starts on start_day, default is <tt>Date.week_start</tt> or <tt>config.week_start</tt> when set.
+    # Week starts on start_day, default is <tt>Date.beginning_of_week</tt> or <tt>config.beginning_of_week</tt> when set.
     def all_week(start_day = Date.beginning_of_week)
       beginning_of_week(start_day)..end_of_week(start_day)
     end