OAPI: return log tail as string, not array of strings

The JavaScript client cannot handle a plain array of strings.

pkg/api/flamenco-openapi.yaml

@@ -444,8 +444,7 @@ paths:
           content:
             text/plain:
               schema:
-                type: array
+                type: string
-                items: { type: string }
         default:
           description: Unexpected error.
           content: