Update plotting.R

[beigelk]

Made change as described in #22 (comment), changed plotting.R as @pedriniedoardo suggested (labs(color = guide_legend(legend_title)) + to labs(color = legend_title) + on line 32), and plot_density() is now working as expected for me.

# Installing my fork of Nebulosa to test `plot_density`
library("devtools")
install_github("beigelk/Nebulosa")

library(Nebulosa)

pbmc = readRDS("~/test_data/pbmc_processed.Rds")
plot_density(pbmc, features = c("LYZ", "CD14"), joint = TRUE)