Remove additional error from error example (#1097)

The example with `Concrete` and `Template` is supposed to highlight that adding properties to a protocol in the implementation of a method is not supported, and does not add to the required signature of the protocol.
However, this also has the problem of `Concrete` not being a `Template` anyway because it is missing that `method`.

pep-0544.txt

@@ -369,6 +369,9 @@ Examples::
           self.name = name
           self.value = value
       
+      def method(self) -> None:
+          return
+
   var: Template = Concrete('value', 42)  # OK
 
 To distinguish between protocol class variables and protocol instance