docs(README): fix link to documentation

Author: woile

docs/README.md

@@ -11,7 +11,7 @@ versions](https://img.shields.io/pypi/pyversions/commitizen.svg?style=flat-squar
 
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-**Documentation**: https://commitizen-tools.github.io/
+**Documentation**: https://commitizen-tools.github.io/commitizen/
 
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