[DOCS] Reformat warning

We should have used the standard warning here because we don't need a title for the admonition

Author: leemthompo

docs/reference/elasticsearch/mapping-reference/mapping-source-field.md

@@ -308,8 +308,7 @@ PUT my-index-000001
 }
 ```
 
-::::{admonition} Disabling the `_source` field
-:class: warning
+::::{warning}
 
 Do not disable the `_source` field, unless absolutely necessary. If you disable it, the following critical features will not be supported:
 
@@ -320,7 +319,6 @@ Do not disable the `_source` field, unless absolutely necessary. If you disable
 * The ability to debug queries or aggregations by viewing the original document used at index time.
 * Potentially in the future, the ability to repair index corruption automatically.
 
-
 ::::
 
 ::::{note}