Prepend should be append in the 30 minute intro

The examples are about adding to the end of the array, not the
beginning.
auto merge of #18748 : carols10cents/rust/prepend-to-append, r=alexcrichton

A most trivial documentation correction. The examples in the intro are all about adding to the end of the array, not the beginning, but this one line says "prepend".

This isn't a very serious problem, it just made me a bit confused when I got to it.