[format.formatter.spec] Add missing include to example (#6636)

The example code refers to `std::string` directly so it should `#include<string>`.

source/utilities.tex

@@ -16981,6 +16981,7 @@
 \begin{example}
 \begin{codeblock}
 #include <format>
+#include <string>
 
 enum color { red, green, blue };
 const char* color_names[] = { "red", "green", "blue" };