Use instructions from opensearch-project#11844 to run the plugin.

Signed-off-by: dblock <dblock@amazon.com>

plugins/api/README.md

@@ -12,25 +12,45 @@ A plugin that returns the OpenSearch API in the OpenAPI format.
 
 ### Run
 
-Modify `gradle/run.gradle`.
+When you run OpenSearch with `./gradlew run -PinstalledPlugins="['api']"` you should see the plugin loaded in the logs.
+
+```
+[2023-09-27T08:35:48,627][INFO ][o.o.p.PluginsService] [runTask-0] loaded plugin [api]
+```
+
+Alternately you can modify `gradle/run.gradle`.
 
 ```gradle
 testClusters {
   runTask {
-    testDistribution = 'archive'
     plugin(':plugins:api')
   }
 }
 ```
 
-    When you run OpenSearch with `./gradlew run` you should see the plugin loaded in the logs.
-
-```
-[2023-09-27T08:35:48,627][INFO ][o.o.p.PluginsService] [runTask-0] loaded plugin [api]
-```
-
-### Test
+### Try
 
 ```sh
 $ curl http://localhost:9200/_plugins/api
 ```
+
+Returns an OpenAPI spec.
+
+```json
+{
+  "openapi": "3.0.1",
+  "info": {
+    "title": "opensearch",
+    "description": "The OpenSearch Project: https://opensearch.org/",
+    "version": "3.0.0-SNAPSHOT"
+  },
+  "paths": {
+    "/_nodes": {
+      "get": {}
+    },
+    "/_cluster/state/:metric": {
+      "get": {}
+    },
+    ...
+  }
+```