docs: can.rst: fix a footnote reference

As stated at:
	http://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#footnotes



A footnote should contain either a number, a reference or
an auto number, e. g.:
        [1], [#f1] or [#].

While using [*] accidentaly works for html, it fails for other
document outputs. In particular, it causes an error with LaTeX
output, causing all books after networking to not be built.

So, replace it by a valid syntax.

Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Jonathan Corbet <corbet@lwn.net>