[docs] update broken link to nightly branch

[miguelcsx]

• The link to the example of implementing operators for a custom type was broken, as it pointed to the main branch.
• Updated the URL to point to the correct nightly branch, ensuring the link now leads to the correct example: https://github.com/modularml/mojo/tree/nightly/examples/operators
• The change was made on the page: https://docs.modular-dev.com/nightly/mojo/manual/operators/#an-example-of-implementing-operators-for-a-custom-type