docs(plugins): add json formating

docs/sources/plugins/datasources.md

@@ -42,13 +42,13 @@ metricFindQuery(options) // used by query editor to get metric suggestions.
 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',
-  targets:
-   [ { refId: 'B', target: 'upper_75' },
-     { refId: 'A', target: 'upper_90' } ],
-  format: 'json',
-  maxDataPoints: 2495 //decided by the panel
+  "range": { "from": "2015-12-22T03:06:13.851Z", "to": "2015-12-22T06:48:24.137Z" },
+  "interval": '5s',
+  "targets":
+   [ { refId: "B", target: "upper_75" },
+     { refId: "A", target: "upper_90" } ],
+  "format": "json",
+  "maxDataPoints": 2495 //decided by the panel
 }
 ```