Update evaluation example in README

[baniasbaabe]

Description

The evaluation in the README.md isn't correct anymore.

1. HierarchicalEvaluation isn't in hierarchicalforecast.core anymore.
2. HierarchicalEvaluation's argument is Y_test_df

The example should be something like this:

from hierarchicalforecast.evaluation import HierarchicalEvaluation

def mse(y, y_hat):
    return np.mean((y-y_hat)**2)

evaluator = HierarchicalEvaluation(evaluators=[mse])
evaluator.evaluate(Y_hat_df=Y_rec_df, Y_test_df=Y_test_df, 
                   tags=tags, benchmark='Naive')

Link

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