Incorrect download plot title

[ErdaradunGaztea]

When passing a character vector of packages to plot_download(), the title is incorrect; this is the result of the following call:

plot_downloads(c("versionsort", "deepdep"))

Clearly, versionsort and deepdep don't have to be related (and they aren't each other's dependencies, not now). Thus, we should generate different title depending on what was passed as an argument.