Fix dummy input docstring typos

[cupkk]

What does this PR do?

Fix four dummpy inputs typos in GENERATE_DUMMY_DOCSTRING. This template is attached to ExporterConfig.generate_dummy_inputs, so the misspellings surface in its generated API documentation.

This is a documentation-only change with no runtime behavior or dependency changes.

Validation

• Parsed optimum/exporters/base.py with Python ast and compiled the source.
• Extracted the generated docstring and checked that dummpy is absent and the corrected parameter descriptions are present.
• Ran git diff --check.

Before submitting

• This PR fixes a typo or improves the docs.
• No documentation or runtime tests need to be added for this text-only correction.

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[echarlaix]

thanks for the fix!

[HuggingFaceDocBuilderDev]

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