[Example] Fix a NaN bug in the flax mlm example
## What does this PR do?

Fix a NaN bug in the flax masked language model example. This is a bug introduced in #9133
The min should be max. Otherwise, we will get a NaN.

## Who can review?
@TevenLeScao  @mfuntowicz 
This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>cc @patrickvonplaten <|||||>Hey @merrymercy - super sorry, I saw the PR too late and it was actually already fixed.<|||||>Thanks for your effort on Jax integration! @patrickvonplaten 
Could you also add some doc for these examples https://github.com/huggingface/transformers/tree/master/examples/flax/language-modeling?