idevicesyslog: Use --syslog-relay in description and man page

Both --syslog-relay and --syslog_relay still work regardless, but
it is more aligned with other long options.

docs/idevicesyslog.1

diff --git a/docs/idevicesyslog.1 b/docs/idevicesyslog.1
index cb91193..5a677b6 100644
--- a/docs/idevicesyslog.1
+++ b/docs/idevicesyslog.1
@@ -39,7 +39,7 @@
 .B \-\-colors
 Force writing colored output, e.g. when using \f[B]\-\-output\f[].
 .TP
-.B \-\-syslog_relay
+.B \-\-syslog\-relay
 Use old syslog_relay service instead of os_trace_relay (iOS 9+).
 
 .SH COMMANDS

tools/idevicesyslog.c

diff --git a/tools/idevicesyslog.c b/tools/idevicesyslog.c
index 830b76a..dc404b0 100644
--- a/tools/idevicesyslog.c
+++ b/tools/idevicesyslog.c
@@ -880,7 +880,7 @@
 		"  -o, --output FILE     write to FILE instead of stdout\n"
 		"                        (existing FILE will be overwritten)\n"
 		"  --colors              force writing colored output, e.g. for --output\n"
-		"  --syslog_relay        force use of syslog_relay service\n"
+		"  --syslog-relay        force use of syslog_relay service\n"
 		"\n"
 		"COMMANDS:\n"
 		"  pidlist               Print pid and name of all running processes.\n"