[lex.ext] Fully-qualify std::size_t in example.

Author: zygoloid

source/lex.tex

@@ -1729,7 +1729,7 @@
 
 \begin{codeblock}
 long double operator "" _w(long double);
-std::string operator "" _w(const char16_t*, size_t);
+std::string operator "" _w(const char16_t*, std::size_t);
 unsigned operator "" _w(const char*);
 int main() {
   1.2_w;      // calls \tcode{operator "" _w(1.2L)}