Correct MKRZero board value

Author: sandeepmistry

boards.txt

@@ -151,7 +151,7 @@ mkrzero.build.mcu=cortex-m0plus
 mkrzero.build.f_cpu=48000000L
 mkrzero.build.usb_product="Arduino MKRZero"
 mkrzero.build.usb_manufacturer="Arduino LLC"
-mkrzero.build.board=SAMD_MKR1000
+mkrzero.build.board=SAMD_MKRZERO
 mkrzero.build.core=arduino
 mkrzero.build.extra_flags=-D__SAMD21G18A__ {build.usb_flags}
 mkrzero.build.ldscript=linker_scripts/gcc/flash_with_bootloader.ld