[DOCS] Fix synced flush request typos (#67446)

Author: jrodewig

docs/reference/indices/synced-flush.asciidoc

@@ -17,13 +17,13 @@ POST /twitter/_flush/synced
 [[synced-flush-api-request]]
 ==== {api-request-title}
 
-`POST /<index>/flush/synced`
+`POST /<index>/_flush/synced`
 
-`GET /<index>/flush/synced`
+`GET /<index>/_flush/synced`
 
-`POST /flush/synced`
+`POST /_flush/synced`
 
-`GET /flush/synced`
+`GET /_flush/synced`
 
 
 [[synced-flush-api-desc]]