Move description of -i MSEC to probing options

The -i MSEC, --interval=MSEC option does not directly affect
the output format, but rather the probe generation, i.e., the
minimum time between probes.  Thus it is more fitting to list
it under "probing options" than under "output options".

src/fping.c

@@ -2943,6 +2943,7 @@ void usage(int is_error)
     fprintf(out, "                      (give start and end IP in the target list, or a CIDR address)\n");
     fprintf(out, "                      (ex. %s -g 192.168.1.0 192.168.1.255 or %s -g 192.168.1.0/24)\n", prog, prog);
     fprintf(out, "   -H, --ttl=N        set the IP TTL value (Time To Live hops)\n");
+    fprintf(out, "   -i, --interval=MSEC  interval between sending ping packets (default: %.0f ms)\n", interval / 1e6);
 #ifdef SO_BINDTODEVICE
     fprintf(out, "   -I, --iface=IFACE  bind to a particular interface\n");
 #endif
@@ -2968,7 +2969,6 @@ void usage(int is_error)
     fprintf(out, "   -d, --rdns         show targets by name (force reverse-DNS lookup)\n");
     fprintf(out, "   -D, --timestamp    print timestamp before each output line\n");
     fprintf(out, "   -e, --elapsed      show elapsed time on return packets\n");
-    fprintf(out, "   -i, --interval=MSEC  interval between sending ping packets (default: %.0f ms)\n", interval / 1e6);
     fprintf(out, "   -n, --name         show targets by name (reverse-DNS lookup for target IPs)\n");
     fprintf(out, "   -N, --netdata      output compatible for netdata (-l -Q are required)\n");
     fprintf(out, "   -o, --outage       show the accumulated outage time (lost packets * packet interval)\n");