Fix Data2VecTextForCasualLM example code documentation
# What does this PR do?

Fixes documentation of Data2VecTextForCasualLM example code, as it is currently importing `Data2VecTextTokenizer`, which does not exist, and the tokenizer is actually `RobertaTokenizer`. At the same time, the model name `"data2vec-base"` does not exist, and it doesn't particularly say to create one locally (with the change, it aims to `"facebook/data2vec-text-base"`).
_The documentation is not available anymore as the PR was closed or merged._