message: small print.hpp doc fix
  According to
diff: @@ -143,7 +143,7 @@ namespace eos {
  *
  * Example:
  * @code
  - * char const *s = "Hello World!";
  + * const char *s = "Hello World!";
  * uint64_t unsigned_64_bit_int = 1e+18;
  * uint128_t unsigned_128_bit_int (87654323456);
  * uint64_t string_as_unsigned_64_bit = N(abcde);
  