Fix typos in 01-index.md

* discoverty -> discovery
* Fixed comment that says to use `getBaseUrl`, instead of the correct `getExternalBaseUrl` to get the external URL of a service.

Signed-off-by: Eric <ericbsatterwhite@gmail.com>

docs/backend-system/core-services/01-index.md

@@ -373,7 +373,7 @@ createBackendPlugin({
         discovery: coreServices.discovery,
       },
       async init({ discovery }) {
-        const url = await discoverty.getBaseUrl('derp'); // can also use discovery.getBaseUrl to retrieve external URL
+        const url = await discovery.getBaseUrl('derp'); // can also use discovery.getExternalBaseUrl to retrieve external URL
         const response = await fetch(`${url}/hello`);
       },
     });