Fix instance mode usage of uniformFloat() in shader.modify() example

[aashu2006]

Resolves #8467

Changes

• Fixed the instance mode example for p5.shader.modify() in the reference.
• Updated the example to use sketch.uniformFloat('b') instead of calling uniformFloat() as a global function.
• This aligns the example with correct instance mode usage, where all p5 APIs must be accessed via the sketch instance.

The issue occurred because the example already uses sketch.createCanvas() and sketch.background(), but missed prefixing uniformFloat() with sketch, causing a runtime error in instance mode.

PR Checklist

• npm run lint passes
• [Inline reference] is included / updated
• [Unit tests] are included / updated (not applicable for documentation change)