Fix graphql env variable name in example (#1592)

Author: TheTyrius

README.md

@@ -156,7 +156,7 @@ HOST: "0.0.0.0"
 PORT: "4040"
 MOUNT_PATH: "/"
 PARSE_DASHBOARD_SERVER_URL: "http://localhost:1337/parse"
-PARSE_DASHBOARD_GRAPHQL_URL: "http://localhost:1337/graphql"
+PARSE_DASHBOARD_GRAPHQL_SERVER_URL: "http://localhost:1337/graphql"
 PARSE_DASHBOARD_MASTER_KEY: "myMasterKey"
 PARSE_DASHBOARD_APP_ID: "myAppId"
 PARSE_DASHBOARD_APP_NAME: "MyApp"