drivers/sdcard: Update doc for ESP8266 to use correct SPI number.

machine.SPI(0) results in ValueError on ESP8266.  SPI(1) is the user
hardware SPI port (or use SPI(-1) for software SPI).

drivers/sdcard/sdcard.py

@@ -14,7 +14,7 @@ Example usage on pyboard:
 Example usage on ESP8266:
 
     import machine, sdcard, os
-    sd = sdcard.SDCard(machine.SPI(0), machine.Pin(15))
+    sd = sdcard.SDCard(machine.SPI(1), machine.Pin(15))
     os.umount()
     os.VfsFat(sd, "")
     os.listdir()