plt.show isn't necessary in Jupyter notebooks

[SJaffa]

programming_with_python/_episodes/03-numpy_essential.md

Line 214 in 19edfb7

plt.show()

This line doesn't change the output as the plot shows by default, so we should either remove it or explain it is there because it is useful in python scripts and programs.

[andrewgait]

This is definitely covered in the Intro to python course, but there's no harm in adding a refresher in this chapter.