Remove assert_select_feed from assert_select_encoded documentation

This documentation came from the assert_select plugin, but the
assert_select_feed method was omitted when the plugin was merged
into Rails in 3142502.

actionpack/lib/action_dispatch/testing/assertions/selector.rb

@@ -341,7 +341,7 @@ def count_description(min, max, count) #:nodoc:
       #
       # ==== Examples
       #   # Selects all bold tags from within the title of an ATOM feed's entries (perhaps to nab a section name prefix)
-      #   assert_select_feed :atom, 1.0 do
+      #   assert_select "feed[xmlns='http://www.w3.org/2005/Atom']" do
       #     # Select each entry item and then the title item
       #     assert_select "entry>title" do
       #       # Run assertions on the encoded title elements
@@ -353,7 +353,7 @@ def count_description(min, max, count) #:nodoc:
       #
       #
       #   # Selects all paragraph tags from within the description of an RSS feed
-      #   assert_select_feed :rss, 2.0 do
+      #   assert_select "rss[version=2.0]" do
       #     # Select description element of each feed item.
       #     assert_select "channel>item>description" do
       #       # Run assertions on the encoded elements.