perf config: Modify size factor of snprintf

According to definition of snprintf, it gets size factor including
null('\0') byte.  So '-1' is not neccessary. Also it will be helpful
unfied style with other cases. (eg. builtin-script.c)

Signed-off-by: Sihyeon Jang <uneedsihyeon@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20181201154603.10093-1-uneedsihyeon@gmail.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>