Outdated docstring for passing labels as dict

[This commit](https://github.com/prometheus/client_python/commit/882d5769da695bee7be5a9806b4b1d4762e23a8a) removes support for passing a dictionary to `labels`. However, the docstring for `labels` still documents passing a dict, rather than keyword arguments. I spent unnecessarily long puzzling over an "Incorrect label count" error from a library that appeared to be using the method as documented.