Fix SRTP s_client/s_server options

The -use_srtp s_client/s_server option is supposed to take a colon
separated string as an argument. In master this was incorrectly set to
expect a filename.

Reviewed-by: Rich Salz <rsalz@openssl.org>
diff --git a/apps/s_server.c b/apps/s_server.c
index a1fcb6e..e7c794c 100644
--- a/apps/s_server.c
+++ b/apps/s_server.c
@@ -949,7 +949,7 @@
      "Set the advertised protocols for the NPN extension (comma-separated list)"},
 #endif
 #ifndef OPENSSL_NO_SRTP
-    {"use_srtp", OPT_SRTP_PROFILES, '<',
+    {"use_srtp", OPT_SRTP_PROFILES, 's',
      "Offer SRTP key management with a colon-separated profile list"},
     {"alpn", OPT_ALPN, 's',
      "Set the advertised protocols for the ALPN extension (comma-separated list)"},