Update order examples to use max instead of avg (#22032)

The use of the avg aggregation for sorting the terms aggregation is not encouraged since it has unbounded error. This changes the examples to use the max aggregation which does not suffer the same issues

docs/reference/aggregations/bucket/terms-aggregation.asciidoc

@@ -297,10 +297,10 @@ Ordering the buckets by single value metrics sub-aggregation (identified by the
         "genres" : {
             "terms" : {
                 "field" : "genre",
-                "order" : { "avg_play_count" : "desc" }
+                "order" : { "max_play_count" : "desc" }
             },
             "aggs" : {
-                "avg_play_count" : { "avg" : { "field" : "play_count" } }
+                "max_play_count" : { "max" : { "field" : "play_count" } }
             }
         }
     }
@@ -316,7 +316,7 @@ Ordering the buckets by multi value metrics sub-aggregation (identified by the a
         "genres" : {
             "terms" : {
                 "field" : "genre",
-                "order" : { "playback_stats.avg" : "desc" }
+                "order" : { "playback_stats.max" : "desc" }
             },
             "aggs" : {
                 "playback_stats" : { "stats" : { "field" : "play_count" } }