update env var names in documentation (#9)

Author: antoineverin

website/docs/index.html.markdown

@@ -25,8 +25,8 @@ provider "elkaliases" {
 Provider settings can be specified via environment variables as follows:
 
 ```shell
-export ELKALIASES_URL=http://localhost:9200
-export ELKALIASES_TOKEN=token
+export ELASTICSEARCH_ENDPOINT=http://localhost:9200
+export ELASTICSEARCH_API_KEY=token
 ```
 
 ## Argument Reference