fix habtm documentation to correct typo

Signed-off-by: NRizwan Reza <rizwanreza@gmail.com>

activerecord/lib/active_record/associations.rb

@@ -1210,7 +1210,7 @@ def belongs_to(association_id, options = {})
       # * <tt>Developer#projects.empty?</tt>
       # * <tt>Developer#projects.size</tt>
       # * <tt>Developer#projects.find(id)</tt>
-      # * <tt>Developer#clients.exists?(...)</tt>
+      # * <tt>Developer#projects.exists?(...)</tt>
       # * <tt>Developer#projects.build</tt> (similar to <tt>Project.new("project_id" => id)</tt>)
       # * <tt>Developer#projects.create</tt> (similar to <tt>c = Project.new("project_id" => id); c.save; c</tt>)
       # The declaration may include an options hash to specialize the behavior of the association.