[Docs] Fix url for feature migration APIs (#86330)

Both GET and POST version of the API have a leading underscore.

docs/reference/migration/apis/feature-migration.asciidoc

@@ -18,9 +18,9 @@ process.
 [[feature-migration-api-request]]
 ==== {api-request-title}
 
-`GET /migration/system_features`
+`GET /_migration/system_features`
 
-`POST /migration/system_features`
+`POST /_migration/system_features`
 
 [[feature-migration-api-prereqs]]
 ==== {api-prereq-title}