Missing color getter in FovsCollection

During the documentation build for the release 0.11.0, the Instrument Trajectory > Instrument field of views page failed to build correctly due to missing color getters.

This is due to a change of get_facecolors() to get_facecolor() in moon-coverage.projections.proj.Projection.xy_collection() in 5bc4b8ac. This change was not propagated to FovsCollection and was not covered by our test suite.