[examples/flax/language-modeling] set loglevel
This PR sets the unset loglevel to a `info` like the rest of flax examples where it wasn't done so.

Fixes a report in https://github.com/huggingface/transformers/pull/14909#issuecomment-1011467609

@patil-suraj, @sgugger 

Thanks so much @stas00 , problem is solved now! Level is not correctly `INFO` during training!<|||||>Thank you for confirming that, @stefan-it.<|||||>Not the main person on the Flax examples so will let @patil-suraj or @patrickvonplaten chime in on the PR :-)