btrfs-progs: doc: fix format of btrfs-replace

btrfs-progs: doc: fix format of btrfs-replace

Current 'man btrfs-replace' is as follows:

========================================================================
...
...
           -f
               force using and overwriting <targetdev> even if it looks like
               containing a valid btrfs filesystem.

               A valid filesystem is assumed if a btrfs superblock is found
               which contains a correct checksum. Devices which are currently
               mounted are never allowed to be used as the <targetdev>. -B::::
               no background replace.
...
...
========================================================================

The format of 'B' option is wrong. So, fix it.

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