docs: Correct custom_labels usage example in README

[newrachael]

What does this do?

Corrects the example code in README.md regarding the usage of the custom_labels parameter. Specifically, it removes the incorrect example showing custom_labels being passed to the Instrumentator constructor and clarifies that it should be passed to individual metric functions via the add() method.

Why do we need it?

The previous example in the "Creating the Instrumentator" section of README.md incorrectly showed custom_labels as a parameter for the Instrumentator constructor. This does not align with the actual code implementation and could confuse users trying to add custom labels to their metrics. This PR fixes the documentation to accurately reflect the correct usage.

Who is this for?

This documentation fix is for all users of the prometheus-fastapi-instrumentator library, especially those looking to add custom labels to their Prometheus metrics.

Linked issues

N/A (If this addresses a specific issue, replace N/A with Resolves #issue_number)

Reviewer notes

This is a straightforward documentation fix to align the README.md examples with the actual code implementation for custom_labels. No code changes are involved.

[andermirik]

👍