Plurality fix; add a comma before however

> Do note that anonymous functions aren't only used with goroutine however.

"goroutine" should be plural here, and adding a comma before the "however" is probably preferred.

> Do note that anonymous functions aren't only used with goroutines, however.

Thanks again Karl! Great, concise book as always. I've been using Go here-and-there for a few months and this has really helped solidify some otherwise difficult concepts.