docs:process:changes: fix version command for btrfs-progs

docs:process:changes: fix version command for btrfs-progs

The command given in the changes.rst document to check the version of
btrfs-progs is:
-> btrfsck
which does not output the version, and according to manual page of the
btrfs-progs the command to check the version of btrfs-progs is:
-> btrfs --version

Add a fix changing the command to check the version of btrfs-progs.

Signed-off-by: Nihar Chaithanya <niharchaithanya@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20241012141425.11852-1-niharchaithanya@gmail.com