btrfs-progs: doc: fix incorrect format of 'l' option in mkfs.btrfs

btrfs-progs: doc: fix incorrect format of 'l' option in mkfs.btrfs

The format of 'l' option in mkfs.btrfs.txt is wrong.
And, when the head of the character string is 65536, the following warning
is displayed.

    $ make
    Making all in Documentation
        [ASCII]  mkfs.btrfs.xml
    asciidoc: WARNING: mkfs.btrfs.xml.tmp1: line 67: list item index: expected 1 got 65536
        [XMLTO]  mkfs.btrfs.8
        [GZ]     mkfs.btrfs.8.gz
    rm mkfs.btrfs.8 mkfs.btrfs.xml

So, fix it.

Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>