avformat/rtpproto: set ttl upper bound to 255

Also remove bogus millisecond reference from help text.
Signed-off-by: Marton Balint <cus@passwd.hu>

libavformat/rtpproto.c

@@ -68,7 +68,7 @@ typedef struct RTPContext {
 #define D AV_OPT_FLAG_DECODING_PARAM
 #define E AV_OPT_FLAG_ENCODING_PARAM
 static const AVOption options[] = {
-    { "ttl",                "Time to live (in milliseconds, multicast only)",                   OFFSET(ttl),             AV_OPT_TYPE_INT,    { .i64 = -1 },    -1, INT_MAX, .flags = D|E },
+    { "ttl",                "Time to live (multicast only)",                                    OFFSET(ttl),             AV_OPT_TYPE_INT,    { .i64 = -1 },    -1, 255,     .flags = D|E },
     { "buffer_size",        "Send/Receive buffer size (in bytes)",                              OFFSET(buffer_size),     AV_OPT_TYPE_INT,    { .i64 = -1 },    -1, INT_MAX, .flags = D|E },
     { "rtcp_port",          "Custom rtcp port",                                                 OFFSET(rtcp_port),       AV_OPT_TYPE_INT,    { .i64 = -1 },    -1, INT_MAX, .flags = D|E },
     { "local_rtpport",      "Local rtp port",                                                   OFFSET(local_rtpport),   AV_OPT_TYPE_INT,    { .i64 = -1 },    -1, INT_MAX, .flags = D|E },