Small fix to website documentation for adding exercises

Lunch break’s over or I’d fix this myself.

“Concept Exercises can always be added later, once you’re track is up-and-running properly.”

should be

"Concept Exercises can always be added later, once your track is up-and-running properly.

“is up-and-running properly” likely should be “is up and running properly” as well, but the you / you’re thing is more black and white.

I’ve opened a PR: