Fix typos

[KevCaz]

Hi @sylvaticus,

Thanks for all your work on this. As I was going through your tutorial I fixed several typos, not sure whether this is still under active development (I understand that there is a MOOC and a different branch of this for your book), but doing the PR is a tiny effort that I thought was worth doing.

[sylvaticus]

Sorry, I quickly saw this pull request. Will stidy it tomorrow...