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!
`Second argument is an array literal`. I am confused on the `[]int{7, 8, 9}...` part because it says here that is an `array literal` but above examples of arrays say an array is `[...]int(3, 1, 5}` and a slice is `[]int{1, 2, 3}` so the `[]int{7, 8, 9}` looks more like a slice to me.