docs: Recommend using "ip" instead of "ifconfig" in CANBUS.md

Some Linux systems do not install ifconfig, while ip should always be
available.  So, update the canbus documentation to recommend that.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/CANBUS.md

@@ -31,7 +31,7 @@ adapter. This is typically done by creating a new file named
 allow-hotplug can0
 iface can0 can static
     bitrate 1000000
-    up ifconfig $IFACE txqueuelen 128
+    up ip link set $IFACE txqueuelen 128
 ```
 
 ## Terminating Resistors
@@ -113,7 +113,7 @@ Some important notes when using this mode:
 allow-hotplug can0
 iface can0 can static
     bitrate 1000000
-    up ifconfig $IFACE txqueuelen 128
+    up ip link set $IFACE txqueuelen 128
 ```
 
 * The "bridge mcu" is not actually on the CAN bus. Messages to and