ts2phc: Fix description of holdover option in man page.

Fix the man page to explain that the LOCKED_STABLE servo state is
enabled by setting the servo_offset_threshold, not
servo_num_offset_values, which is already enabled by default.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>

ts2phc.8

@@ -283,7 +283,7 @@ The holdover interval, specified in seconds. When the ToD information stops
 working (e.g. GNSS receiver lost its fix), ts2phc is allowed for the specified
 interval to continue synchronizing the target clock as long as the servo is in
 the SERVO_LOCKED_STABLE state. The servo state needs be enabled by the
-\fBservo_num_offset_values\fP option. The holdover is not supported with the
+\fBservo_offset_threshold\fP option. The holdover is not supported with the
 \fB-a\fP option and when \fBts2phc.extts_polarity\fP is set to \fIboth\fP.
 The default is 0 (disabled).