avfilter/avf_showspectrum: make colors for log scale more user friendly

Previosly output was almost useless because background noise, due to
windowing function picked and which is not actually present in audio,
had too much brightness.
Now output of sine wave matches more with SoX.

Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavfilter/avf_showspectrum.c

@@ -380,7 +380,7 @@ static int plot_spectrum_column(AVFilterLink *inlink, AVFrame *insamples)
                 a = cbrt(a);
                 break;
             case LOG:
-                a = 1 + log10(FFMAX(FFMIN(1, a), 1e-6)) / 6; // zero = -120dBFS
+                a = 1 + log10(FFMAX(FFMIN(1, a), 1e-6)) / 5; // zero = -120dBFS
                 break;
             default:
                 av_assert0(0);