Extension’s loadComplete call doesn't match the official guideline

[muzimuzhi]

Currently the loadComplete call in packaged siunix.js is

MathJax.Ajax.loadComplete("[Contrib]/siunitx/siunitx.js")

The path the call loads, "[Contrib]/siunitx/siunitx.js", is inconsistent with

• the guidelines (#using-third-party-extensions) provided by official MathJax-third-party-extensions repo
• the related documentation (#custom-extension-path-configuration) provided by official MathJax Doc,
• and the practices popular third party extension mhchem obey.

Following the docs and practices, the path loadComplete loads should be "[siunitx]/siunitx.js".

[burnpanck]

That must have been a change post the migration out of the MathJax-third-party-extensions repo. I didn't have time to spend on this project since then. If you make a PR, I'll gladly accept it.