sx: correct the usage of ps

There appears to be no indication according to the POSIX synopsis[1] of
the ps command that it shall take a PID argument as positional.  The
rest of the document doesn't suggest such a case either.

What the document does provide is the `-p' flag to specify a list of
PIDs with which ps will narrow the results.

1. https://pubs.opengroup.org/onlinepubs/9699919799/utilities/ps.html

sx

@@ -19,7 +19,7 @@ cleanup() {
 }
 
 stty=$(stty -g)
-tty=$(ps -o tty= $$)
+tty=$(ps -o tty= -p $$)
 tty=${tty#tty}
 
 cfgdir=${XDG_CONFIG_HOME:-$HOME/.config}/sx