Update installation command in README.md

Adjust default pypi install name

Author: SchrodingersGat

plugin_creator/template/{{ cookiecutter.plugin_name }}/README.md

@@ -13,7 +13,7 @@
 To install manually via the command line, run the following command:
 
 ```bash
-pip install {{ cookiecutter.plugin_slug }}
+pip install inventree-{{ cookiecutter.plugin_slug }}
 ```
 
 ## Configuration