Added an example of exception situation on Array#sample docs

activesupport/lib/active_support/core_ext/array/random_access.rb

@@ -5,10 +5,11 @@ class Array
   # If +n+ is passed and its value is less than 0, it raises an +ArgumentError+ exception.
   # If the value of +n+ is equal or greater than 0 it returns <tt>[]</tt>.
   #
-  #   [1,2,3,4,5,6].sample    # => 4
-  #   [1,2,3,4,5,6].sample(3) # => [2, 4, 5]
-  #              [].sample    # => nil
-  #              [].sample(3) # => []
+  #   [1,2,3,4,5,6].sample     # => 4
+  #   [1,2,3,4,5,6].sample(3)  # => [2, 4, 5]
+  #   [1,2,3,4,5,6].sample(-3) # => ArgumentError: negative sample number
+  #              [].sample     # => nil
+  #              [].sample(3)  # => []
   def sample(n=nil)
     return self[Kernel.rand(size)] if n.nil?
     n = n.to_int