docs(api): remove "async" to header getter

.changeset/purple-pans-shout.md

@@ -577,7 +577,7 @@ const instance = new XrpcClient(
     service: () => 'http://my-service.com',
     headers: {
       'my-header': () => 'my-value',
-      'my-ignored-header': async () => null, // ignored
+      'my-ignored-header': () => null, // ignored
     },
   },
   [