Update README.md to use correct links to plugins directory.

Looks like the plugins directory changed from /plugins at some point to /coreplugins so this is a simple update to a few links in the readme.

README.md

@@ -236,9 +236,9 @@ WebODM by itself is just a user interface (see [below](#odm-nodeodm-webodm-what)
 
 Small customizations such as changing the application colors, name, logo, or addying custom CSS/HTML/Javascript can be performed directly from the Customize -- Brand/Theme panels within WebODM. No need to fork or change the code.
 
-More advanced customizations can be achieved by writing [plugins](https://github.com/OpenDroneMap/WebODM/tree/master/plugins). This is the preferred way to add new functionality to WebODM since it requires less effort than maintaining a separate fork. The plugin system features server-side [signals](https://github.com/OpenDroneMap/WebODM/blob/master/app/plugins/signals.py) that can be used to be notified of various events, a ES6/React build system, a dynamic [client-side API](https://github.com/OpenDroneMap/WebODM/tree/master/app/static/app/js/classes/plugins) for adding elements to the UI, a built-in data store, an async task runner, a GRASS engine, hooks to add menu items and functions to rapidly inject CSS, Javascript and Django views.
+More advanced customizations can be achieved by writing [plugins](https://github.com/OpenDroneMap/WebODM/tree/master/coreplugins). This is the preferred way to add new functionality to WebODM since it requires less effort than maintaining a separate fork. The plugin system features server-side [signals](https://github.com/OpenDroneMap/WebODM/blob/master/app/plugins/signals.py) that can be used to be notified of various events, a ES6/React build system, a dynamic [client-side API](https://github.com/OpenDroneMap/WebODM/tree/master/app/static/app/js/classes/plugins) for adding elements to the UI, a built-in data store, an async task runner, a GRASS engine, hooks to add menu items and functions to rapidly inject CSS, Javascript and Django views.
 
-For plugins, the best source of documentation currently is to look at existing [code](https://github.com/OpenDroneMap/WebODM/tree/master/plugins). If a particular hook / entrypoint for your plugin does not yet exist, [request it](https://github.com/OpenDroneMap/WebODM/issues). We are adding hooks and entrypoints as we go.
+For plugins, the best source of documentation currently is to look at existing [code](https://github.com/OpenDroneMap/WebODM/tree/master/coreplugins). If a particular hook / entrypoint for your plugin does not yet exist, [request it](https://github.com/OpenDroneMap/WebODM/issues). We are adding hooks and entrypoints as we go.
 
 To create a plugin simply copy the `plugins/test` plugin into a new directory (for example, `plugins/myplugin`), then modify `manifest.json`, `plugin.py` and issue a `./webodm.sh restart`.