Fixed aggregation by "gender" request example.

`gender.keyword` should be used instead of just `gender` or we
get an error `Fielddata is disabled on text fields by default. Set
fielddata=true on [gender] in order to load fielddata in memory by
uninverting the inverted index. Note that this can however use

Closes #20535
significant memory.`

docs/reference/getting-started.asciidoc

@@ -1059,7 +1059,7 @@ curl -XPOST 'localhost:9200/bank/_search?pretty' -d '
       "aggs": {
         "group_by_gender": {
           "terms": {
-            "field": "gender"
+            "field": "gender.keyword"
           },
           "aggs": {
             "average_balance": {