Fix ViTForMaskedImageModeling example in documentation
Following https://github.com/huggingface/transformers/pull/22152, `logits` is not the right output key name.

cc @alaradirik @amyeroberts 
_The documentation is not available anymore as the PR was closed or merged._<|||||>@fxmarty I am just going to open a PR, but you are too fast! Thanks.<|||||>@ydshieh It could be I missed it elsewhere, so feel free to push here / do an other PR!<|||||>You have to check your CircleCI setting however. The CI has issue to run in this PR.<|||||>OK, nothing missed, but as the CI has some problem, I am going to open PR.<|||||>I will make sure you are a contributor in #22186 @fxmarty . Close this one as mentioned above.<|||||>Ah yes I did not configure SSO with CircleCI, maybe that's the issue.