Fix a typo

Author: zhenya-1007

man/man7/plumb.7

@@ -254,7 +254,7 @@ port of the rule set or the entire rule set will be skipped.
 .B client
 If no application has the port open, the arguments to a
 .B plumb
-.B start
+.B client
 rule specify a shell program to run in response to the message.
 The message will be held, with the supposition that the program
 will eventually open the port to retrieve it.