Minor typos fixed in thread sync tutorial.

Hi Sam,
You probably won't remember me, I met you at a CamJam Earlier in the year. Thought I'd get up to speed on the new sonic pi, as I'm going to run an after school club down here in Cornwall.
This must be the worlds smallest pull request - fixed two minor typos. If this approach works, I'll make more minor fixes in my github and continue with the pull request approach, unless you'd prefer patches to be submitted another way?
Thanks
Graham
P.S. Keep up the good work!, I'm now hooked on live coding, the Monday Blues example is awsome.