edited AvgAggregation doc

docs/Aggregation/Avg.md

@@ -10,7 +10,7 @@ A single-value metrics aggregation that computes the average of numeric values t
 ```JSON
 {
     "aggregations": {
-        "agg_avg_grade": {
+        "avg_grade": {
             "avg": {
                 "field": "grade"
             }
@@ -31,4 +31,4 @@ $search->addAggregation($avgAggregation);
 $queryArray = $search->toArray();
 ```
 
-[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-avg-aggregation.html
\ No newline at end of file
+[1]: https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-metrics-avg-aggregation.html