no more 'add_spectrum' in gwpy/cli/coherence.py

Created by: young-min

When running 'gwpy-plot coherence' under a recent git version of gwpy, an error arose as follows,

AttributeError: 'FrequencySeriesPlot' object has no attribute 'add_spectrum'

I figured out that 'add_spectrum' was renamed 'add_frequencyseries' and you can find a log for this change as follows, https://github.com/gwpy/gwpy/commit/843443b650f81b4e3f2c658cc6591e9e45592c14

The simple solution for this issue is to fix the line 152 in 'gwpy/gwpy/cli/coherence.py' such as self.plot.add_spectrum(coh) --> self.plot.add_frequencyseries(coh)

I found only one use of 'add_spectrum' in 'gwpy/gwpy/cli/coherence.py' , but I'm not sure there are more uses.