Reduce LR for TF MLM example test
The TF MLM example test was a little flakey, depending on the exact shuffling order of the dataset. I reduced the LR to 1e-4 and it seems to be consistently around 36 final perplexity now (compared to the test threshold of 42).

Cc @sgugger 
_The documentation is not available anymore as the PR was closed or merged._