Fix intervals section of auto date-histogram docs (#41203)

This section should be at the same sub-level as other sections in the
auto date-histogram docs, otherwise it is rendered on to another page
and is confusing for users to understand what it's in reference to.

Author: jasontedor

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

@@ -89,7 +89,7 @@ Response:
 --------------------------------------------------
 // TESTRESPONSE[s/\.\.\./"took": $body.took,"timed_out": false,"_shards": $body._shards,"hits": $body.hits,/]
 
-=== Intervals
+==== Intervals
 
 The interval of the returned buckets is selected based on the data collected by the 
 aggregation so that the number of buckets returned is less than or equal to the number