docs(plugins): json formating

docs/sources/plugins/datasources.md

@@ -43,7 +43,7 @@ Request object passed to datasource.query function
 ```json
 {
   "range": { "from": "2015-12-22T03:06:13.851Z", "to": "2015-12-22T06:48:24.137Z" },
-  "interval": '5s',
+  "interval": "5s",
   "targets": [
     { "refId": "B", "target": "upper_75" },
     { "refId": "A", "target": "upper_90" }