Corrected some of the slide effects.  Exported to `.pptx` and `.pdf` formats.
On branch master, changes to be committed:
	new file:   Linux_intro_extra.pdf
	new file:   Linux_intro_extra.pptx
	modified:   Linux_intro_presentation.odp
	new file:   Linux_intro_presentation.pdf
	new file:   Linux_intro_presentation.pptx