rm .default() in argument for set up function guide (#7910)

Author: josefaidt

src/pages/[platform]/build-a-backend/functions/set-up-function/index.mdx

@@ -89,7 +89,7 @@ const schema = a.schema({
   sayHello: a
     .query()
     .arguments({
-      name: a.string().default("World"),
+      name: a.string(),
     })
     .returns(a.string())
     .handler(a.handler.function(sayHello)),