Fix --serial help
Make explicit that --serial excepts a parameter.
@@ -86,7 +86,7 @@ static void usage(const char *arg0) {
" This flag forces to render all frames, at a cost of a\n"
" possible increased latency.\n"
"\n"
- " -s, --serial\n"
+ " -s, --serial serial\n"
" The device serial number. Mandatory only if several devices\n"
" are connected to adb.\n"
"\n"
