[diff.expr] Replace "For example" with \begin{example} environment

Author: eisenwave

source/compatibility.tex

@@ -3047,7 +3047,7 @@
 taking the address of a dereferenced null or past-the-end pointer value
 is well-defined in C (and yields the original pointer value),
 but results in undefined behavior in \Cpp{}.
-For example:
+\begin{example}
 \begin{codeblock}
 void f() {
   char *p = nullptr;
@@ -3057,6 +3057,7 @@
   int *q = &a[5];       // well-defined in C, undefined behavior in \Cpp{}
 }
 \end{codeblock}
+\end{example}
 \rationale
 Consistent treatment of lvalues in \Cpp{}.
 \effect