WL-3139 Add PowerPoint to TurnItIn instructions

TurnItIn now accepts powerpoint files so add this to the list of
submission instructions.
WL-3139 Remove PPT from instructions

TurnItIn also accepts things like pptx so let's not specify this single
file type explicitly.
Merge pull request #8 from ox-it/WL-3139

WL-3139 Add PowerPoint to TurnItIn instructions