Update neural-network.md

[langfan94]

change input r,g,b number => string

Dear ml5 community,

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

A good PR 🌟

→ Step 1: Which branch are you submitting to? 🌲

Development (for new features or updates)

→ Step 2: Describe your Pull Request 📝

Update input r,g,b number => string

A great PR 🌟🌟

→ Step 3: 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 4: Screenshots or Relevant Documentation 🖼

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

[joeyklee]

@langfan94 - Hi! Thanks for taking the time to make these documentation suggestions. Can you explain why there is a need to change the numbers to strings?

From the documentation changes I'm not sure why this is necessary. Any clarifications and additional details would be great! Thanks!