Update start server script

scripts/start_server

 #!/bin/bash
 
 usage() {
-  echo "Usage: $0 [-t] [-c NB_PACKETS]" 1>&2;
+  echo "Usage: $0 [-t NB_PACKETS]" 1>&2;
   exit 1;
 }
 
-while getopts "c:t" opt; do
+while getopts "t:" opt; do
   case "${opt}" in
-    c )
-        nb_packets=${OPTARG}
-        ;;
     t )
         use_tcpdump=1
+        nb_packets=${OPTARG}
         ;;
     * )
         usage