Fix track selection header copy

@AlasdairSwan and @dianakhuang, I noticed a pretty visible error in the copy displayed in the header on the track selection page. Currently, after enrolling in, for example, FooX's Course 6, it reads "Congrats! You are now enrolled in the audit track FooX's Course 6" [sic]. This should instead read something like "Congrats! You are now enrolled in FooX's Course 6." (Excluding mention of any enrollment track in this copy is also more consistent with the rest of the page, since track selection is meant to occur on this page, not before it; "Now choose your course track" is displayed immediately below this line.)

I'm not sure if this issue is urgent enough to warrant a PR against the RC, but Griff suggested that we should fix it quickly, if possible; this change is also very low risk.

@dan-f, FYI.