[libc++][NFC] Qualify mention of va_list with std:: in documentation

Author: ldionne

libcxx/docs/UsingLibcxx.rst

@@ -187,7 +187,7 @@ and ``operator delete``. For example:
   #include <version> // must include any libc++ header before defining the function (C compatibility headers excluded)
 
   void std::__libcpp_verbose_abort(char const* format, ...) {
-    va_list list;
+    std::va_list list;
     va_start(list, format);
     std::vfprintf(stderr, format, list);
     va_end(list);