AP_Menu: fix wrong printf format for printf

"%S" is used for wide string, but we are passing a char*. Use lowercase
in this case to remove warnings like this:

libraries/AP_InertialSensor/AP_InertialSensor.cpp: In member function
'bool AP_InertialSensor::calibrate_accel(AP_InertialSensor_UserInteract*, float&, float&)':
libraries/AP_InertialSensor/AP_InertialSensor.cpp:620:61: warning:
format '%S' expects argument of type 'wchar_t*', but argument 3 has type 'const char*' [-Wformat=]
                 "Place vehicle %S and press any key.\n", msg);
                                                             ^

libraries/AP_Menu/AP_Menu.cpp

@@ -84,7 +84,7 @@ Menu::_check_for_input(void)
 void
 Menu::_display_prompt(void)
 {
-    _port->printf("%S] ", _prompt);
+    _port->printf("%s] ", _prompt);
 }
 
 // run the menu
@@ -231,7 +231,7 @@ Menu::_help(void)
     _port->println("Commands:");
     for (i = 0; i < _entries; i++) {
 		hal.scheduler->delay(10);
-        _port->printf("  %S\n", _commands[i].command);
+        _port->printf("  %s\n", _commands[i].command);
 	}
 }