Fix broken link in contributor_docs/contributing_to_the_p5js_reference.md

[aleannab]

Most appropriate sub-area of p5.js?

• Accessibility
• Color
• Core/Environment/Rendering
• Data
• DOM
• Events
• Image
• IO
• Math
• Typography
• Utilities
• WebGL
• Build process
• Unit testing
• Internationalization
• Friendly errors
• Other (specify if possible)

p5.js version

v1.10.0 (latest)

Web browser and version

Firefox v130.0, though would affect all.

Operating system

Windows, though would affect all.

Steps to reproduce this

Steps:

1. Go to: https://github.com/processing/p5.js/blob/main/contributor_docs/contributing_to_the_p5js_reference.md
2. Click on respective link (listed in table below).
3. Get a 404 error.

Solutions:

Replace links to the suggested links below.

Source link text:	Current link:	Suggested link:
web accessibility contributor documentation	https://p5js.org/contributor-docs/#/web_accessibility?id=user-generated-accessible-canvas-descriptions	https://p5js.org/contribute/web_accessibility/#describe

Note: Although the original link goes to the web accessibility docs, since the respective text references describe(), I thought it would be better to link directly to the describe section as specified above.

For more on describe() visit the web accessibility contributor documentation.

Let me know if the above works, and I'll submit a PR.