Fix copy & paste error

README.md

@@ -25,7 +25,7 @@ For the development version:
 git clone https://github.com/jonasw234/merge_image_metadata
 cd merge_image_metadata
 python3 setup.py install
-pip3 install -r dev-requirements.txt
+pip3 install -r dev-requirements.in
 ```
 
 For normal usage do the same but don’t include the last line or use `[pipx](https://pypi.org/project/pipx/)` and install with