typo fix

README.md

@@ -55,7 +55,7 @@ dns01
   .set({
     identifier: { value: 'foo.example.com' },
     wildcard: false,
-    dnsHost: '_acme-challenge.foo.example.com'
+    dnsHost: '_acme-challenge.foo.example.com',
     dnsAuthorization: 'xxx_secret_xxx'
   })
   .then(function () {