docs: remove removed examples guidelines reference from PR template (#83043)
## For Maintainers
### What?
Removed the references to the now-deleted examples guidelines from the
PR template. Specifically, removed the line about
```
The "examples guidelines" are followed from our contributing doc https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md
```
since that file and folder were deleted in PR
[#77050](https://github.com/vercel/next.js/pull/77050).
### Why?
The `/examples` folder and its `adding-examples.md` file were removed,
so the reference in the PR template to those guidelines is outdated and
misleading. Keeping those references causes confusion for contributors
by pointing to non-existent documentation.
### How?
- Edited the PR template file.
- Removed the outdated examples guidelines reference line.
- Left other template content unchanged to preserve existing guidance.
This ensures the PR template stays accurate, up-to-date, and helpful for
contributors.
---------
Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>