get_abi.pl: fix xref boundaries

There are some issues with the regex that seeks for What:
cross references: basically, it is mis-identifying the start
and the end boundaries of the regex, which causes :ref: to
be inseerted for the wrong symbols at the wrong places.

Fix it.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/79a14d2518499b76931b5f29c50979987108152d.1616668017.git.mchehab+huawei@kernel.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>