Clarify how to enable syntax highlighting

[0xazure]

Rephrase and correct missing word, and provide a full example of applying Go syntax highlighting to a Markdown code block.

Previously, the phrasing was:

All you need to do is to let rainbow.js the language of the highlighted code

I think that should have read

All you need to do is to let rainbow.js know the language of the highlighted code

but I rephrased it a bit to simplify the written instructions and then provided a full example using a Go code sample.

I just grabbed the sample off the Go homepage, let me know if there's a different code sample we could use instead.