Fix the type for "slices" in the Delete By Query REST API specification (#51792)

The previous patch in c1d9966d35d incorrectly set the `type` to `number|auto`,
which is incorrect — the "polymorphic" type, denoted with the `|` sign,
should contain only other types, ie. number, string, bool, etc.

Fixes #51535

rest-api-spec/src/main/resources/rest-api-spec/api/delete_by_query.json

@@ -173,9 +173,9 @@
         "description":"The throttle for this request in sub-requests per second. -1 means no throttle."
       },
       "slices":{
-        "type":"number|auto",
+        "type":"number|string",
         "default":1,
-        "description":"The number of slices this task should be divided into. Defaults to 1, meaning the task isn't sliced into subtasks."
+        "description":"The number of slices this task should be divided into. Defaults to 1, meaning the task isn't sliced into subtasks. Can be set to `auto`."
       }
     },
     "body":{