Update the MappingStructure constant in the docs to match the enum values

(cherry picked from commit f09837e)

Author: marko-bekhta

extensions/hibernate-search-standalone-elasticsearch/runtime/src/main/java/io/quarkus/hibernate/search/standalone/elasticsearch/runtime/HibernateSearchStandaloneBuildTimeConfig.java

@@ -269,7 +269,7 @@ interface MappingConfig {
          * Associations between entities must be bi-directional:
          * specifying the inverse side of associations through `@AssociationInverseSide` *is required*,
          * unless reindexing is disabled for that association through `@IndexingDependency(reindexOnUpdate = ...)`.
-         * `tree`::
+         * `document`::
          * Entities indexed through Hibernate Search are the root of a document,
          * i.e. an indexed entity "owns" other entities it references through associations,
          * which *cannot* be updated independently of the indexed entity.