drivers/sdcard: Update SD mounting example code for ESP8266.

drivers/sdcard/sdcard.py

@@ -15,9 +15,8 @@ Example usage on ESP8266:
 
     import machine, sdcard, os
     sd = sdcard.SDCard(machine.SPI(1), machine.Pin(15))
-    os.umount()
+    os.mount(sd, '/sd')
-    os.VfsFat(sd, "")
+    os.listdir('/')
-    os.listdir()
 
 """