append() function is not mentioned in Slices section

Slices are a fundamental datatype in Go, and lack of mention of how they are updated seems like a significant oversight. Hopefully, this change adequately resolves this oversight. Thanks!
Sorry, I have been away and swamped. Yes, that is incorrect on my part, it is a slice literal not an array literal, all slice literals use an empty set `[]`. I can fix this quickly. Does this look OK otherwise?