Rename plot_continuous_var.R to plot_continuous_var.r

Requires a lowercase r in the file extension other wise this error (in French here) is thrown: "impossible d'ouvrir le fichier 'plot_continuous_var.r' : Aucun fichier ou dossier de ce type" . It reads "Impossible to open the file 'plot_continuous_var.r': No file or folder by that name."