fix(readme): support tensorflow >= 2.5.1

Author: nglehuy

README.md

@@ -5,7 +5,7 @@
   <img alt="GitHub" src="https://img.shields.io/github/license/TensorSpeech/TensorFlowASR?logo=apache&logoColor=green">
 </a>
 <img alt="python" src="https://img.shields.io/badge/python-%3E%3D3.6-blue?logo=python">
-<img alt="tensorflow" src="https://img.shields.io/badge/tensorflow-%3E%3D2.3.0-orange?logo=tensorflow">
+<img alt="tensorflow" src="https://img.shields.io/badge/tensorflow-%3E%3D2.5.1-orange?logo=tensorflow">
 <a href="https://pypi.org/project/TensorFlowASR/">
   <img alt="PyPI" src="https://img.shields.io/pypi/v/TensorFlowASR?color=%234285F4&label=release&logo=pypi&logoColor=%234285F4">
 </a>