mouseY reference first example issue fixed

[shourysingh07]

Resolves issue #583 of p5.js website repo.

Changes:

• Added a horizontal black line that moves up and down following the mouse's Y-position in the first example for mouseY, where it was previously missing.

Screenshots of the change:

Before:

https://github.com/user-attachments/assets/2fc37a41-9db6-4056-8d4f-b81b091ba2a3

After:

https://github.com/user-attachments/assets/d53b91ec-37cb-4824-89a3-c0d4d1f534d4