Blog Post: "How To Train" points to code that doesn't exist

**Link to blog post**: [How to train a new language model from scratch using Transformers and Tokenizers](https://huggingface.co/blog/how-to-train)

In section 3 of the article:

> We will now train our language model using the [run_language_modeling.py](https://github.com/huggingface/transformers/blob/master/examples/language-modeling/run_language_modeling.py) script from transformers....

That link is dead.

1. It looks like that the `run_language_modeling.py` file was moved from its previous location (which was at `examples/language-modeling/run_language_modeling.py`) to the new location at `examples/contrib/legacy/language-modeling/run_language_modeling.py` ([link](https://github.com/huggingface/transformers/blob/master/examples/contrib/legacy/language-modeling/run_language_modeling.py)) in [this commit](https://github.com/huggingface/transformers/commit/cdc48ce92ddf50e7ad871376be651638268b2e9a) in Oct 2020. However it looks like the file was deleted from the second location too.
2. From the second location the file was moved to `examples/legacy/run_language_modeling.py` in [this commit](https://github.com/huggingface/transformers/commit/783d7d2629e97c5f0c5f9ef01b8c66410275c204#diff-82eb66d4b0af8440b3f92f69c0fdc33cba55e0858fa348966a9c834b8c089f71) in Dec 2020.

It would be nice if: (1) this blog post was either marked as legacy/out of date, or (2) the link is updated to [this](https://github.com/huggingface/transformers/blob/master/examples/legacy/run_language_modeling.py), assuming support for this still exists.