Wrong model is used in example, should be character instead of subword model
# What does this PR do?

A canine.rst fix.
In the original Google repo for CANINE there was mixup in the model names in the README.md, which was fixed 2 weeks ago. Since this transformer model was created before, it probably resulted in wrong use in this example in canine.rst.

s = subword, c = character

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## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

## Who can review?

@sgugger

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It's not easy to make a file the styler is happy with, haha.