Fix ostream.h build

include/fmt/ostream.h

@@ -142,7 +142,7 @@ inline void vprint(std::basic_ostream<Char> &os,
   \endrst
  */
 template <typename S, typename... Args>
-inline typename std::enable_if<internal::is_format_string<S>::value>::type
+inline typename std::enable_if<internal::is_string<S>::value>::type
 print(std::basic_ostream<FMT_CHAR(S)> &os, const S &format_str,
       const Args & ... args) {
   internal::checked_args<S, Args...> ca(format_str, args...);