fix(openapi): avoid double slash for base

Author: pi0

src/runtime/routes/openapi.ts

@@ -6,14 +6,15 @@ import type {
   ParameterObject,
   PathsObject,
 } from "openapi-typescript";
+import { joinURL } from "ufo";
 import { handlersMeta } from "#internal/nitro/virtual/server-handlers";
 import { useRuntimeConfig } from "#internal/nitro";
 
 // Served as /_nitro/openapi.json
 export default eventHandler((event) => {
   const base = useRuntimeConfig()?.app?.baseURL;
 
-  const url = getRequestURL(event).origin + base;
+  const url = joinURL(getRequestURL(event).origin, base);
 
   return <OpenAPI3>{
     openapi: "3.0.0",