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 <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Acked-by: Jonathan Corbet <[e-mail]>

Documentation/networking/can.rst

···
 transmission and reception of media dependent frames. Due to the
 arbitration on the CAN bus the transmission of a low prio CAN-ID
 may be delayed by the reception of a high prio CAN frame. To
-reflect the correct [*]_ traffic on the node the loopback of the sent
+reflect the correct [#f1]_ traffic on the node the loopback of the sent
 data has to be performed right after a successful transmission. If
 the CAN network interface is not capable of performing the loopback for
 some reason the SocketCAN core can do this task as a fallback solution.
···
 the RT-SocketCAN group the loopback optionally may be disabled for each
 separate socket. See sockopts from the CAN RAW sockets in :ref:`socketcan-raw-sockets`.
 
-.. [*] you really like to have this when you're running analyser
+.. [#f1] you really like to have this when you're running analyser
        tools like 'candump' or 'cansniffer' on the (same) node.