Remove "stupid" from active record

The language in the docs is clearer without saying MySQL is stupid. The
test doesn't need stupid in the value to be used - an incorrect value is
incorrect.

activerecord/lib/arel/visitors/mysql.rb

@@ -75,7 +75,7 @@ module Arel # :nodoc: all
         end
         alias :prepare_delete_statement :prepare_update_statement
 
-        # MySQL is too stupid to create a temporary table for use subquery, so we have
+        # MySQL doesn't automatically create a temporary table for use subquery, so we have
         # to give it some prompting in the form of a subsubquery.
         def build_subselect(key, o)
           subselect = super

activerecord/test/cases/scoping/default_scoping_test.rb

@@ -362,7 +362,7 @@ class DefaultScopingTest < ActiveRecord::TestCase
 
   def test_unscope_errors_with_invalid_value
     assert_raises(ArgumentError) do
-      Developer.includes(:projects).where(name: "Jamis").unscope(:stupidly_incorrect_value)
+      Developer.includes(:projects).where(name: "Jamis").unscope(:incorrect_value)
     end
 
     assert_raises(ArgumentError) do