ovl: Kconfig: Fix spelling mistake "undelying" -> "underlying"

There is a spelling mistake in a Kconfig description. Fix it.
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/Kconfig

@@ -96,7 +96,7 @@ config OVERLAY_FS_XINO_AUTO
 	depends on 64BIT
 	help
 	  If this config option is enabled then overlay filesystems will use
-	  unused high bits in undelying filesystem inode numbers to map all
+	  unused high bits in underlying filesystem inode numbers to map all
 	  inodes to a unified address space.  The mapped 64bit inode numbers
 	  might not be compatible with applications that expect 32bit inodes.