RDoc: fix wrong model name :inverse_of with :belongs_to [ci skip]

[vrybas]

There's a typo in ActiveRecord associations RDocs.
Wrong Taggable model name, instead of Tagging in example of using
option :inverse_of with :belongs_to association.

Commit where typo was introduced:
91fd651#diff-39001423802a8470dba9c931e66e101eR11

First it appears in activerecord/CHANGELOG in example of :inverse_of
usage:

    class Post < ActiveRecord::Base
      has_many :taggings
      has_many :tags, :through => :taggings
    end

    class Tagging < ActiveRecord::Base
      belongs_to :post
      belongs_to :tag, :inverse_of => :tagging # :inverse_of must be set!
    end

    class Tag < ActiveRecord::Base
      has_many :taggings
      has_many :posts, :through => :taggings
    end

    post = Post.first
    tag = post.tags.build :name => "ruby"
!>  tag.save # will save a Taggable linking to the post

The last line should be

    tag.save # will save a Tagging linking to the post

The same typo appears in
activerecord/lib/active_record/associations.rb.
The association name is given as :inverse_of => :taggings, but class
name is Taggable.

    #   @post = Post.first
    #   @tag = @post.tags.build :name => "ruby"
    #   @tag.save
    #
!>  # The last line ought to save the through record (a <tt>Taggable</tt>). This will only work if the
    # <tt>:inverse_of</tt> is set:
    #
!>  #   class Taggable < ActiveRecord::Base
    #     belongs_to :post
!>  #     belongs_to :tag, :inverse_of => :taggings
    #   end

This PR fixes model name.

[robin850]

Thanks!