Update orderBy documentation to include using multiple calls for multiple columns

[ismailnagib]

Knex.js supports using multiple orderBy calls to use multiple columns in the SQL ORDER BY clause. This pull request adds an example of this behavior to the documentation, to show that such a method is indeed supported by Knex.js.

This is a link to a unit test from the Knex.js repository demonstrating this behavior:
https://github.com/knex/knex/blob/master/test/unit/query/builder.js#L3434-L3456

[ismailnagib]

Hi @OlivierCavadenti, I'm not sure if it's okay for me to mention you, but it seems that you're the one who merged the previous PRs. Please kindly take a look, it's only 2 lines of changes. Thank you!

[rluvaton]

The website repo has been moved inside the https://github.com/knex/knex repo. Please open a PR there instead: (the website will be under the docs folder)