btrfs-progs: doc replace, sort subcommands alphabetically

Signed-off-by: David Sterba <dsterba@suse.cz>

Documentation/btrfs-replace.txt

@@ -21,6 +21,9 @@ management.
 
 SUBCOMMAND
 ----------
+*cancel* <mount_point>::
+Cancel a running device replace operation.
+
 *start* [-Bfr] <srcdev>|<devid> <targetdev> <path>::
 Replace device of a btrfs filesystem.
 +
@@ -61,9 +64,6 @@ Print status and progress information of a running device replace operation.
 print once instead of print continuously until the replace
 operation finishes (or is canceled)
 
-*cancel* <mount_point>::
-Cancel a running device replace operation.
-
 EXIT STATUS
 -----------
 *btrfs replace* returns a zero exit status if it succeeds. Non zero is