Update doc/source/guide/helioviewer.rst
Update doc/source/guide/helioviewer.rst

Updated to make a note of the need to install OpenJPEG.
dpshelio/sunpy,dpshelio/sunpy,Alex-Ian-Hamilton/sunpy,Alex-Ian-Hamilton/sunpy,mjm159/sunpy,dpshelio/sunpy,mjm159/sunpy,Alex-Ian-Hamilton/sunpy