[diff.expr] Update incorrect comment about C (cplusplus#8933)

arr2 is a constraint violation in C, rather than being valid.

Author: Halalaluyafail3

source/compatibility.tex

@@ -3067,8 +3067,8 @@
 \begin{codeblock}
 int arr1[5];
 int arr2[5];
-int same = arr1 == arr2;        // valid C, ill-formed C++
-int idem = arr1 == +arr2;       // valid in both C and C++
+int same = arr1 == arr2;        // valid C, ill-formed \Cpp{}
+int idem = arr1 == +arr2;       // valid \Cpp{}, constraint violation in C
 \end{codeblock}
 \end{example}
 \howwide