Fix duplicate externalUrl problem (#24)

Fixes #20 
```release-note
delete duplicate externalUrl in item's link
```

src/main/java/run/halo/feed/FeedServiceImpl.java

@@ -167,7 +167,7 @@ private Mono<ServerResponse> postListResultToXmlServerResponse(
                 }
                 var itemBuilder = RSS2.Item.builder()
                     .title(post.getSpec().getTitle())
-                    .link(feedContext.externalUrl.toString() + permalink)
+                    .link(permalink)
                     .pubDate(post.getSpec().getPublishTime())
                     .guid(post.getStatusOrDefault().getPermalink());