Revert the guide after removing varargs from `in?`.

guides/source/active_support_core_extensions.md

@@ -476,12 +476,11 @@ NOTE: Defined in `active_support/core_ext/kernel/reporting.rb`.
 
 ### `in?`
 
-The predicate `in?` tests if an object is included in another object or a list of objects. An `ArgumentError` exception will be raised if a single argument is passed and it does not respond to `include?`.
+The predicate `in?` tests if an object is included in another object. An `ArgumentError` exception will be raised if the argument passed does not respond to `include?`.
 
 Examples of `in?`:
 
 ```ruby
-1.in?(1,2)          # => true
 1.in?([1,2])        # => true
 "lo".in?("hello")   # => true
 25.in?(30..50)      # => false