change activerecord query conditions example to avoid 'type' as column name

'Type' is a reserved column for STI.  Changed conditions example to
avoid using that column name as an example.  The example isn't
STI-related (and mentioning STI here is needless clutter), so changing
to avoid accidentally encouraging users to use 'type' as a column name
for other purposes.

activerecord/lib/active_record/base.rb

@@ -115,8 +115,8 @@ module ActiveRecord #:nodoc:
   # When joining tables, nested hashes or keys written in the form 'table_name.column_name'
   # can be used to qualify the table name of a particular condition. For instance:
   #
-  #   Student.joins(:schools).where(:schools => { :type => 'public' })
-  #   Student.joins(:schools).where('schools.type' => 'public' )
+  #   Student.joins(:schools).where(:schools => { :category => 'public' })
+  #   Student.joins(:schools).where('schools.category' => 'public' )
   #
   # == Overwriting default accessors
   #