the evil DR MERTO strikes again!

fixes adafruit#38

Author: ladyada

variants/metro_m4/variant.h

@@ -16,8 +16,8 @@
   Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
 
-#ifndef _VARIANT_MERTO_M4_
-#define _VARIANT_MERTO_M4_
+#ifndef _VARIANT_METRO_M4_
+#define _VARIANT_METRO_M4_
 
 // The definitions here needs a SAMD core >=1.6.10
 #define ARDUINO_SAMD_VARIANT_COMPLIANCE 10610
@@ -267,5 +267,5 @@ extern Uart Serial1;
 #define SERIAL_PORT_HARDWARE        Serial1
 #define SERIAL_PORT_HARDWARE_OPEN   Serial1
 
-#endif /* _VARIANT_MERTO_M4_ */
+#endif /* _VARIANT_METRO_M4_ */