Fix arity warning for template handlers

Mainly to help with knowning which template is reponsible for the
warning.

handler.class # => Class

handler.to_s # => Coffee::Rails::TemplateHandler

Before:

Change:
  >> Class#call(template)
To:
  >> Class#call(template, source)

After:

Change:
  >> Coffee::Rails::TemplateHandler.call(template)
To:
  >> Coffee::Rails::TemplateHandler.call(template, source)

actionview/lib/action_view/template/handlers.rb

@@ -45,12 +45,12 @@ def register_template_handler(*extensions, handler)
 
         unless params.find_all { |type, _| type == :req || type == :opt }.length >= 2
           ActiveSupport::Deprecation.warn <<~eowarn
-          Single arity template handlers are deprecated.  Template handlers must
+          Single arity template handlers are deprecated. Template handlers must
           now accept two parameters, the view object and the source for the view object.
           Change:
-            >> #{handler.class}#call(#{params.map(&:last).join(", ")})
+            >> #{handler}.call(#{params.map(&:last).join(", ")})
           To:
-            >> #{handler.class}#call(#{params.map(&:last).join(", ")}, source)
+            >> #{handler}.call(#{params.map(&:last).join(", ")}, source)
           eowarn
           handler = LegacyHandlerWrapper.new(handler)
         end