fix model table cell text alignment
# What does this PR do?

The current new model table in [index.mdx](https://github.com/ydshieh/transformers/blob/master/docs/source/index.mdx) left align table cell texts. This PR makes them center-aligned.

On master:

![Capture](https://user-images.githubusercontent.com/2521628/147839482-9ed2520e-3622-4f66-bdfe-3c98f2d8e655.JPG)

On this PR [index.mdx](https://github.com/ydshieh/transformers/blob/center-align-model-table/docs/source/index.mdx):

![Capture2](https://user-images.githubusercontent.com/2521628/147839492-a4a2ee48-e8ab-49c5-86f3-71a4e7b1e05a.JPG)

(Hope centralized instead of decentralized is intended in this context 🤗 )

## Who can review?

@sgugger  @LysandreJik 