Documentation: Fix size parameter for snprintf

cppcheck reports this error:

Documentation/laptops/hpfall.c:33]: (error)
 Dangerous usage of 'devname' (strncpy doesn't always 0-terminate it)

The terminating '\0' is needed for the global char array unload_heads_path,
so never write the last array entry (which is initially '\0').

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>