correct trailing space on +R usermode documentation

irc/help.go

   +a  |  User is marked as being away. This mode is set with the /AWAY command.
   +i  |  User is marked as invisible (their channels are hidden from whois replies).
   +o  |  User is an IRC operator.
-  +R  |  User only accepts messages from other registered users. 
+  +R  |  User only accepts messages from other registered users.
   +s  |  Server Notice Masks (see help with /HELPOP snomasks).
   +Z  |  User is connected via TLS.
   +B  |  User is a bot.