Fix DecisionTransformerConfig doctring

[joaoareis]

What does this PR do?

Currently, the default values for the n_layer and n_head parameters in the DecisionTransformerConfig are set to 3 and 1, respectively. However, the docstring says that the default value is 12 for both. This PR fixes the docstring to reflect the correct default values.

Default values:

transformers/src/transformers/models/decision_transformer/configuration_decision_transformer.py

Lines 120 to 121 in f2d2880

	n_layer=3,
	n_head=1,

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• Did you write any new necessary tests?

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CC @edbeeching

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[amyeroberts]

Thanks for fixing this!