[expr.sizeof] use constexpr member in example

Author: Eisenwave

source/expressions.tex

@@ -5004,7 +5004,7 @@
 \begin{codeblock}
 template<class... Types>
 struct count {
-  static const std::size_t value = sizeof...(Types);
+  static constexpr std::size_t value = sizeof...(Types);
 };
 \end{codeblock}
 \end{example}