Fix not matching return types in Strategy docs

[adam-narozniak]

Issue

We keep the types parameters as type hints in code and in docs. In Strategy the docs return types do not reflect the type hints return types.

Description

aggregate_fit and aggregate_evaluate docs return types do not match the type hints in the code

Proposal

Fix the docs return types.

Explanation

The type hints are correct based on the code functionality therefore, the docs types need to be changed.