Update README.md

Fixing minor mistake with reference to cloning git repository

README.md

@@ -492,7 +492,7 @@ In this case, `message_foo_counter` has `tag`, `hostname`, `key` and `data_type`
 Checkout repository and setup.
 
 ```
-$ git clone git://github.com/fluent/fluent-plugin-prometheus
+$ git clone git://github.com/fluent/fluent-plugin-prometheus.git
 $ cd fluent-plugin-prometheus
 $ bundle install --path vendor/bundle
 ```