docs(readme): fix 'pip install Commitizen' to 'pip install commitizen'

Signed-off-by: Adrian DC <[email protected]>

docs/README.md

@@ -49,7 +49,7 @@ descriptive commits.
 To make commitizen available in your system
 
 ```bash
-pip install --user -U Commitizen
+pip install --user -U commitizen
 ```
 
 ### Python project