docs: fix references to docs/devel/s390-dasd-ipl.rst

docs: fix references to docs/devel/s390-dasd-ipl.rst

Commit cc3d15a5ea ("docs: rstfy s390 dasd ipl documentation")
converted docs/devel/s390-dasd-ipl.txt to docs/devel/s390-dasd-ipl.rst.

We still have several references to the old file, so let's fix them
with the following command:

  sed -i s/s390-dasd-ipl.txt/s390-dasd-ipl.rst/ \
      $(git grep -l docs/devel/s390-dasd-ipl.txt)

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20210517151702.109066-6-sgarzare@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>