Fix ssl_server2 sample application prompt

FIx the type of server_addr parameter from %d to %s.
Issue reported by Email by Bei Jin
diff --git a/programs/ssl/ssl_server2.c b/programs/ssl/ssl_server2.c
index 2c5d9eb..5b4d634 100644
--- a/programs/ssl/ssl_server2.c
+++ b/programs/ssl/ssl_server2.c
@@ -254,7 +254,7 @@
 #define USAGE \
     "\n usage: ssl_server2 param=<>...\n"                   \
     "\n acceptable parameters:\n"                           \
-    "    server_addr=%%d      default: (all interfaces)\n"  \
+    "    server_addr=%%s      default: (all interfaces)\n"  \
     "    server_port=%%d      default: 4433\n"              \
     "    debug_level=%%d      default: 0 (disabled)\n"      \
     "    nbio=%%d             default: 0 (blocking I/O)\n"  \