README: fix typo FAT_ENV_DEV_AND_PART

The actual define symbol is FAT_ENV_DEVICE_AND_PART

Signed-off-by: Nicolae Rosia <Nicolae_Rosia@Mentor.com>
diff --git a/README b/README
index 47bc215..25cad2f 100644
--- a/README
+++ b/README
@@ -4110,7 +4110,7 @@
 
          Define this to a string that is the name of the block device.
 
-       - FAT_ENV_DEV_AND_PART:
+       - FAT_ENV_DEVICE_AND_PART:
 
          Define this to a string to specify the partition of the device. It can
          be as following: