[DOCS] Fix hard-coded mapping type doc link (#33319)

Relates to elastic/docs#2521

This PR fixes a hard-coded docmentation link that will cease to work when we move away from "master" URLS:

> 17:07:03 INFO:build_docs:  /tmp/docsbuild/target_repo/html/en/beats/devguide/6.0/metricset-details.html contains broken links to:
17:07:03 INFO:build_docs:   - en/elasticsearch/reference/master/mapping-types.html

NOTE: This fix needs to be backported all the way to 6.0

docs/devguide/metricset-details.asciidoc

@@ -100,7 +100,7 @@ include::../../metricbeat/module/mysql/_meta/fields.yml[]
 +
 * `metricbeat/module/{module}/{metricset}/_meta/fields.yml`: Contains all fields definitions retrieved by the metricset.
 As field types, each field must have a core data type
-https://www.elastic.co/guide/en/elasticsearch/reference/master/mapping-types.html#_core_datatypes[supported by elasticsearch]. Here's a very basic example that shows one group from the MySQL `status` metricset:
+{ref}/mapping-types.html#_core_datatypes[supported by elasticsearch]. Here's a very basic example that shows one group from the MySQL `status` metricset:
 +
 [source,yaml]
 ----