[diff.basic] Use enum example instead of example outdated since C99

C99 subclause 6.2.7, "Compatible type and composite type", had this
sentence:
> If one is declared with a tag, the other shall be declared with the
> same tag.

Author: hubert-reinterpretcast

source/compatibility.tex

@@ -2916,11 +2916,10 @@
 
 \diffref{basic.types}
 \change
-C allows ``compatible types'' in several places, \Cpp{} does not.\\
+C allows mixing between ``compatible types'' in several places where \Cpp{} does not.\\
 For example,
-otherwise-identical \keyword{struct} types with different tag names
-are ``compatible'' in C but are distinctly different types
-in \Cpp{}.
+enumerated types are ``compatible'' with their underlying types in C but, in \Cpp{},
+enumerations are types distinct from their underlying types.
 \rationale
 Stricter type checking is essential for \Cpp{}.
 \effect