btrfs: update kconfig title

The Kconfig title does not make much sense after the cleanup of
CONFIG_EXPERIMENTAL option, align the wording with other filesystems.
Signed-off-by: NDavid Sterba <dsterba@suse.cz>
Signed-off-by: NJosef Bacik <jbacik@fusionio.com>

fs/btrfs/Kconfig

 config BTRFS_FS
-	tristate "Btrfs filesystem Unstable disk format"
+	tristate "Btrfs filesystem support"
 	select LIBCRC32C
 	select ZLIB_INFLATE
 	select ZLIB_DEFLATE