Update object-detector.md

[juilie]

URLs to json files will not work, and will end up using Coco as a fallback.

Dear ml5 community,

I'm making a Pull Request(PR). Please see the details below.

A good PR 🌟

→ Step 1: Describe your Pull Request 📝

Fixing a Bug? Adding an Update? Submitting a New Feature? Does it introduce a breaking change?

A great PR 🌟🌟

→ Step 2: Share a Relevant Example 🦄

Here's some example code or a demonstration of my feature as a part of this pull request, a separate pull request, in the https://editor.p5js.org, or codepen/jsfiddle/etc...

The best PR 🌟🌟🌟

→ Step 3: Screenshots or Relevant Documentation 🖼

Here's some helpful screenshots and/or documentation of the new feature