examples: minor fixes in flax example readme
Hi,

this PR introduces some minor fixes in the FLAX LM doc.

It mainly prevents an:

```
In [6]: tokenizer.save("./")
---------------------------------------------------------------------------
Exception                                 Traceback (most recent call last)
<ipython-input-6-7d2c76b50951> in <module>
----> 1 tokenizer.save("./")

~/dev/lib/python3.8/site-packages/tokenizers/implementations/base_tokenizer.py in save(self, path, pretty)
    334                 A path to the destination Tokenizer file
    335         """
--> 336         return self._tokenizer.save(path, pretty)
    337 
    338     def to_str(self, pretty: bool = False):

Exception: Is a directory (os error 21)
```

error, because `.save()` function of the `ByteLevelBPETokenizer` instance expects a file name.