Documentation: correct example restore from bundle

Documentation: correct example restore from bundle

Because the bundle created in the example does not record HEAD, "git
clone" will not check out the files to the working tree:

    $ git clone pr.bundle q/
    Cloning into 'q'...
    Receiving objects: 100% (619/619), 13.52 MiB | 18.74 MiB/s, done.
    Resolving deltas: 100% (413/413), done.
    warning: remote HEAD refers to nonexistent ref, unable to checkout.

Avoid alarming the readers by adding "-b master" to the example.  A
better fix may be to arrange the bundle created in the earlier step
to record HEAD, so that it can be cloned without this workaround.

Signed-off-by: Brilliantov Kirill Vladimirovich <brilliantov@inbox.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>