Docs: fix incorrect namespace mention

[klmz]

The docs mention the wrong namespace for annotations. The namespace is set here I think: https://github.com/guydc/gateway/blob/e54b00f4e227e0e7d65bc6dbecf5b6d1f991dd7e/internal/gatewayapi/route.go#L35

It might be that i misunderstood the documentation and i'm mixing things up. If so, ill be happy to write a separate section with this example.

What type of PR is this?
Small correction to the docs.

What this PR does / why we need it:
It adds the correct namespace, and a small example of how to use it.

Which issue(s) this PR fixes:

Release Notes: No

[codecov]

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