Grammar fixes for Object#in? documentation.

activesupport/lib/active_support/core_ext/object/inclusion.rb

 class Object
   # Returns true if this object is included in the argument. Argument must be
-  # any object which respond to +#include?+. Usage:
+  # any object which responds to +#include?+. Usage:
   #
   #   characters = ["Konata", "Kagami", "Tsukasa"]
   #   "Konata".in?(characters) # => true
   #
-  # This will throw an ArgumentError if the supplied argument doesnt not respond
+  # This will throw an ArgumentError if the argument doesn't respond
   # to +#include?+.
   def in?(another_object)
     another_object.include?(self)