Correct cross reference in at_pid(9) manual page.

Author: petterreinholdtsen

docs/man/man9/at_pid.9

@@ -232,4 +232,4 @@ welcome to add a parameter that will change the behavior, or to write their own
 version of \fBat_pid\fR. However, the default behavior should not be changed.
 
 .SH SEE ALSO
-\fBat_pid\fR(9)
+\fBpid\fR(9)