Examples for excel_new_pal do not generate results

[mnelsonr]

Example code for excel_new_pal needs to be updated

• to reference ggthemes_data$excel$themes for names and fix typo / parenthesis placement in call to show_col

i.e: inst/examples/ex-excel_new_pal.R

for (i in names(ggthemes::ggthemes_data$excel$themes)) {
    scales::show_col(excel_new_pal(theme = i)(6))
}

Happy to create a PR if that would be helpful