quick docs fix to the zap example

[konradjniemiec]

This example doesn't compile—I get: missing dataset name

[lukasmalkmus]

Hi @konradjniemiec 👋

Thanks for the contribution! What you're describing is actually explained a couple of lines before: https://github.com/axiomhq/axiom-go/pull/311/files#diff-65932b4d21b82ce597f2aa3c53c235560d9da11836ed720eb5ca8355d4678859R23-R25.

Do you think this is not very obvious and we should improve it? Like do just one code block or something similar? What about other code examples like the ones in examples or GoDoc?