Docs: Setting defined custom preset

[saloiofun]

Describe the bug

The custom preset should be passed to preset

- this.config.theme.set(MyPreset);
+ this.config.theme.set({ preset: MyPreset });

Example:
https://stackblitz.com/edit/stackblitz-starters-nmlpfw?file=src%2Fmain.ts,package.json

Environment

PrimeNG v18 Online docs
https://v18.primeng.org/theming#definepreset

Reproducer

No response

Angular version

18.0.0

PrimeNG version

18.0.0

Build / Runtime

Angular CLI App

Language

TypeScript

Node version (for AoT issues node --version)

20.0.0

Browser(s)

No response

Steps to reproduce the behavior

No response

Expected behavior

No response