There's also a cool trick you can do to spell-check in the course itself: with the course open in a browser window, open the developer tools by pressing F12, go to the console, then execute the following:
This command makes all text within the page editable which, in turn, should trigger the spell-checker - in Firefox anyway. In Chrome you have to force a spell-check either by changing a word or via Edit > Spelling and Grammar > Check document now.
Needless to say this trick doesn't work in IE11 ;-) It may work in Edge, I have no way of knowing since the Edge developer tools have been broken for me for several months now.
And obviously if you do edit the text those changes will not be saved!