Add -P to man (#612)

nnn.1

@@ -21,6 +21,7 @@
 .Op Ar -l
 .Op Ar -n
 .Op Ar -p file
+.Op Ar -P key
 .Op Ar -Q
 .Op Ar -r
 .Op Ar -R
@@ -103,6 +104,9 @@ supports the following options:
 .Fl "p file"
         copy (or \fIpick\fR) selection to file, or stdout if file='-'
 .Pp
+.Fl "P key"
+        specify plugin key to run
+.Pp
 .Fl Q
         disable confirmation on quit with multiple contexts active
 .Pp