DOC: Remove duplicated Series.dt.normalize from docs
`Series.dt.normalize` is listed both as an attribute and as a method. Removing the attribute instance, since it's a method.

> Does this need to be backported to 2.2.x?

No big difference, but if we do the docs of the 2.2 version won't have the duplicate. So I added it to the milestone. Thanks @datapythonista 