:bug: fix small model card bugs
Author: nateraw
Pull request: yes

# What does this PR do?

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- `model_index` ➡️ `model-index`
- `metric` ➡️ `metrics`
- Metrics Dict ➡️ List of Metrics Dicts

These changes fix problem of user-provided evaluation metrics not showing up on model pages pushed to hub from trainer.

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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## Who can review?

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
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- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

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Comments:
good catch and thanks for working on this

Heres a repo I created with it: [nateraw/vit-base-beans-demo-v3](https://huggingface.co/nateraw/vit-base-beans-demo-v3)