DOC: Fix "kwargs" description for .assign() (pandas-dev#60588)

Fix "kwargs" description for .assign()

"kwargs" isn't a dict; the keyword arguments are *converted* to a dict.
Secondly, keyword arguments are strings by definition.

pandas/core/frame.py

@@ -5009,7 +5009,7 @@ def assign(self, **kwargs) -> DataFrame:
 
         Parameters
         ----------
-        **kwargs : dict of {str: callable or Series}
+        **kwargs : callable or Series
             The column names are keywords. If the values are
             callable, they are computed on the DataFrame and
             assigned to the new columns. The callable must not