reference_psd: fix irrelevant off-by-one bug

- in PSDFirKernel.psd_to_linear_phase_whitening_fir_kernel(), this fixes an off-by-one bug in the calculation of the sample rate.  note that the sample rate is rounded to the nearest integer, so this patch has no effect on the numerical value, but it's better for the expression to be correct for the sake of someone reading it.

gstlal/python/reference_psd.py

@@ -477,7 +477,7 @@ class PSDFirKernel(object):
 		#
 
 		data = psd.data.data / 2
-		sample_rate = 2 * int(round(psd.f0 + len(data) * psd.deltaF))
+		sample_rate = 2 * int(round(psd.f0 + (len(data) - 1) * psd.deltaF))
 
 		#
 		# remove LAL normalization