Add missed angle bracket to the help output

As one was lost in the nats-io#3536

Signed-off-by: Alexander Kuleshov <[email protected]>

main.go

@@ -38,7 +38,7 @@ Server Options:
     -c, --config <file>              Configuration file
     -t                               Test configuration and exit
     -sl,--signal <signal>[=<pid>]    Send signal to nats-server process (ldm, stop, quit, term, reopen, reload)
-                                     pid> can be either a PID (e.g. 1) or the path to a PID file (e.g. /var/run/nats-server.pid)
+                                     <pid> can be either a PID (e.g. 1) or the path to a PID file (e.g. /var/run/nats-server.pid)
         --client_advertise <string>  Client URL to advertise to other servers
         --ports_file_dir <dir>       Creates a ports file in the specified directory (<executable_name>_<pid>.ports).