Edit err message and comment in `test_model_is_small`
Author: connor-henderson
Pull request: yes

# What does this PR do?
Tiny error message and comment update to bring into alignment with 1M param max mentioned in https://github.com/huggingface/transformers/pull/24824#issue-1804886584

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_25087). All of your documentation changes will be reflected on that endpoint.