Nokia RX-51: Fix documentation how to enable UBI support

Disable UBI fastmap support which is not supported by original Maemo 5
kernel and explicitly set UBI BEB limit to 10%, which is the value used by
original Maemo 5 kernel. U-Boot default value is 20%.

Signed-off-by: Pali Rohár <pali@kernel.org>
diff --git a/doc/board/nokia/rx51.rst b/doc/board/nokia/rx51.rst
index 061fe76..7c6647b 100644
--- a/doc/board/nokia/rx51.rst
+++ b/doc/board/nokia/rx51.rst
@@ -158,8 +158,7 @@
 
     CONFIG_CMD_UBI=y
     CONFIG_CMD_UBIFS=y
-    CONFIG_MTD_UBI_FASTMAP=y
-    CONFIG_MTD_UBI_FASTMAP_AUTOCONVERT=1
+    CONFIG_MTD_UBI_BEB_LIMIT=10
 
 Run in QEMU
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