Revert "update doc, assert_template also accepts false"

This reverts commit 9b1c077c.

my bad.
`assert_template` only accepts a String, Symbol, Hash, Regexp or nil.

`false`(layout: false) is provided as a Hash.

actionpack/lib/action_controller/test_case.rb

@@ -170,7 +170,7 @@ def assert_template(options = {}, message = nil)
             "Expected no partials to be rendered"
         end
       else
-        raise ArgumentError, "assert_template only accepts a String, Symbol, Hash, Regexp, nil or false"
+        raise ArgumentError, "assert_template only accepts a String, Symbol, Hash, Regexp, or nil"
       end
     end
   end