[DOCS] Fix field name to match description (#81621)

docs/reference/mapping/dynamic/field-mapping.asciidoc

@@ -88,7 +88,7 @@ PUT my-index-000001
 
 PUT my-index-000001/_doc/1 <1>
 {
-  "create": "2015/09/02"
+  "create_date": "2015/09/02"
 }
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