Minor nit: typo in specs for ADRF65xx

Both the ADRF6520 and ADRF6518 have spectral noise density specified in dBV/Hz.  It should be dBV/sqrt(Hz).  The error has been noted (but not corrected) for the 6518.  The same error cropped up in the 6520.  It is correct for the ADRF6510 and ADRF6516.