[FLAX] Minor fixes in CLM example (#12914)

* readme: fix retrieval of vocab size for flax clm example

* examples: fix flax clm example when using training/evaluation files

examples/flax/language-modeling/README.md

@@ -211,7 +211,7 @@ from transformers import GPT2Config
 
 model_dir = "./norwegian-gpt2"  # ${MODEL_DIR}
 
-config = GPT2Config.from_pretrained("gpt2", resid_pdrop=0.0, embd_pdrop=0.0, attn_pdrop=0.0, vocab_size=tokenizer.vocab_size)
+config = GPT2Config.from_pretrained("gpt2", resid_pdrop=0.0, embd_pdrop=0.0, attn_pdrop=0.0, vocab_size=tokenizer.get_vocab_size())
 config.save_pretrained(model_dir)
 ```