Avoid negative, don't refer to directive

Co-authored-by: Lee Byron <[email protected]>

Author: benjie

spec/Section 3 -- Type System.md

@@ -947,8 +947,7 @@ IsValidImplementation(type, implementedType):
        2. Let {implementedFieldType} be the return type of {implementedField}.
        3. {IsValidImplementationFieldType(fieldType, implementedFieldType)} must
           be {true}.
-    6. If the `@deprecated` directive is not applied to {implementedField}, then
-       the `@deprecated` directive must not be applied to {field}.
+    6. If {field} is deprecated then {implementedField} must also be deprecated.
 
 IsValidImplementationFieldType(fieldType, implementedFieldType):