date: Use an explicit time format string for default option

This is to prepare for removing the time zone from DateTime's default
format string. The date utility on most system show time zone by default
so let's keep that.

Userland/Utilities/date.cpp

@@ -60,7 +60,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
     } else if (print_rfc_3339) {
         outln("{}", date.to_string("%Y-%m-%d %H:%M:%S%:z"));
     } else {
-        outln("{}", date.to_string());
+        outln("{}", date.to_string("%Y-%m-%d %H:%M:%S %Z"));
     }
     return 0;
 }