Hotfix for #353 (#367)

Change error message

src/panel/graph_panel/graph_ctrl.ts

@@ -763,7 +763,7 @@ class GraphCtrl extends MetricsPanelCtrl {
 
   private async _getDatasourceRequest() {
     if(this._datasourceRequest === undefined) {
-      throw new Error('Datasource is not set. If it`s Grafana test-datasource - it`s not supported');
+      throw new Error('Datasource is not set. If it`s Grafana-test-datasource then it`s not supported');
     }
     if(this._datasourceRequest.type === undefined) {
       const datasource = await this._getDatasourceByName(this.panel.datasource);