Update downsample-data-stream.asciidoc to remove average from downsampling statistics in documentation

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/indices/downsample-data-stream.asciidoc

@@ -12,8 +12,8 @@ For the most up-to-date API details, refer to {api-es}/group/endpoint-data-strea
 --
 
 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 in the downsample index.
@@ -123,4 +123,4 @@ to aggregate the original time series index. For example, `60m` produces a
 document for each 60 minute (hourly) interval. This follows standard time
 formatting syntax as used elsewhere in {es}.
 +
-NOTE: Smaller, more granular intervals take up proportionally more space.
+NOTE: Smaller, more granular intervals take up proportionally more space.