Updates decremented .readall() to .read() (#377)

Author: pycommatthew

content/firmwareapi/pycom/machine/sd.md

@@ -33,7 +33,7 @@ f = open('/sd/test.txt', 'w')
 f.write('Testing SD card write operations')
 f.close()
 f = open('/sd/test.txt', 'r')
-f.readall()
+f.read()
 f.close()
 ```