Add missing header in put_data_lifecycle rest-api-spec (#116292) (#116370)

docs/changelog/116292.yaml

@@ -0,0 +1,5 @@
+pr: 116292
+summary: Add missing header in `put_data_lifecycle` rest-api-spec
+area: Data streams
+type: bug
+issues: []

rest-api-spec/src/main/resources/rest-api-spec/api/indices.put_data_lifecycle.json

@@ -7,7 +7,8 @@
     "stability":"stable",
     "visibility":"public",
     "headers":{
-      "accept": [ "application/json"]
+      "accept": [ "application/json"],
+      "content_type": ["application/json"]
     },
     "url": {
       "paths": [