DOC: correct currenmoduel for API stringmethods

Now we use Series.str this is not in the submodule anymore

Author: jorisvandenbossche

doc/source/api.rst

@@ -517,8 +517,6 @@ String handling
 strings and apply several methods to it. These can be acccessed like
 ``Series.str.<function/property>``.
 
-.. currentmodule:: pandas.core.strings
-
 .. autosummary::
    :toctree: generated/
    :template: autosummary/accessor_method.rst