Update ggsave code to save plots in plotting chapter

[nkandhari]

In the plotting chapter (https://github.com/MonashDataFluency/python-workshop-base/blob/master/workshops/docs/modules/plotting_with_ggplot.md), update the ggsave code for saving the plot. The existing code doesn't work.

Existing code: prints an error
ggsave(filename="plot1.png", plot=plt1, device='png', dpi=300, height=25, width=25)
Updated code:
ggsave(plt1, filename="plot1.png", width = 25, height = 25, dpi= 300)

[nkandhari]

The same changes must be made in the half-day and the full-day workshops.