Update reference from DateHistogram to Histogram (#26169)

DateHistogram was renamed, but java api docs were not updated. This commit updates a reference to DateHistogram to the new general Histogram class.

docs/java-api/search.asciidoc

@@ -116,7 +116,7 @@ SearchResponse sr = client.prepareSearch()
 
 // Get your facet results
 Terms agg1 = sr.getAggregations().get("agg1");
-DateHistogram agg2 = sr.getAggregations().get("agg2");
+Histogram agg2 = sr.getAggregations().get("agg2");
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 See <<java-aggs,Aggregations Java API>>