Clarify documentation on config file

[jl-stellar]

Under "Custom SVGO configuration", there are 2 code examples for config files.
The first one is a complete config file; however, the second one is missing the module.exports = before the curly braces.

So if someone just copies the entire block and pastes it into the svgo.config.js file, it won't work.

(Especially since the second code block is prefixed with the comment "For the record, here's the default configuration we use:")

It would be nice if this could be fixed, I don't think it will make the documentation less readable.