fix typo in readme (#5399)

Co-authored-by: Ebey Abraham <ebeyabraham@microsoft.com>

README.md

@@ -680,7 +680,7 @@ python3 -m pip install -r requirements.txt
 python3 convert.py models/mymodel/
 
 # [Optional] for models using BPE tokenizers
-python convert.py models/mymodel/ --vocabtype bpe
+python convert.py models/mymodel/ --vocab-type bpe
 
 # quantize the model to 4-bits (using Q4_K_M method)
 ./quantize ./models/mymodel/ggml-model-f16.gguf ./models/mymodel/ggml-model-Q4_K_M.gguf Q4_K_M