Update ilm-downsample.asciidoc

Heya Team! 

I came across a support case where it was discovered "average" statistic has no longer been included in downsampling for a while now. It is possible for customers to calculate it or run the average aggregation against the data, but it will not be displayed as one of the other statistics. 

It was removed from the documentation for one specific page, but all the other references were leftover. Trying to clean those leftovers up to avoid customer confusion. 

See #110189 for original removal.

Author: ZachBrisson-Elastic

docs/reference/ilm/actions/ilm-downsample.asciidoc

@@ -5,8 +5,8 @@
 Phases allowed: hot, warm, cold.
 
 Aggregates a time series (TSDS) index and stores
-pre-computed statistical summaries (`min`, `max`, `sum`, `value_count` and
-`avg`) for each metric field grouped by a configured time interval. For example,
+pre-computed statistical summaries (`min`, `max`, `sum`, and `value_count`)
+for each metric field grouped by a configured time interval. For example,
 a TSDS index that contains metrics sampled every 10 seconds can be downsampled
 to an hourly index. All documents within an hour interval are summarized and
 stored as a single document and stored in the downsample index.
@@ -52,4 +52,4 @@ PUT _ilm/policy/datastream_policy
     }
   }
 }
-----
+----