fix ppc-distribution docs for argument usage & correct test function in test-ppc-distribution

[behramulukir]

In ppc-distribution, there is some insufficient information in the documentation, which should be updated. Also, test-ppc-distribution has a typo, which makes tests run the wrong function while testing. Here is a list of the things to be fixed:

documentation:

• binwidth, bins, and breaks arguments are passed to ggdist::stat_dots and ggplot2::geom_area in addition to gggplot2::geom_hist
• freq argument is used in frequency polygons in addition to histograms

testing:

• in test-ppc-distribution, test_that("ppc_dens,pp_hist,ppc_freqpoly,ppc_boxplot return ggplot objects”) function uses ppc_dens function instead of ppd_dens function at the part where it tests ppd versions of the functions

[jgabry]

Great, thanks for finding these issues.