fix broken rss guid and link

lib/generate-rss.js

@@ -2,9 +2,9 @@ import siteMetadata from '@/data/siteMetadata'
 
 const generateRssItem = (post) => `
   <item>
-    <guid>${siteMetadata.siteUrl}${post.slug}</guid>
+    <guid>${siteMetadata.siteUrl}/${post.slug}</guid>
     <title>${post.title}</title>
-    <link>${siteMetadata.siteUrl}${post.slug}</link>
+    <link>${siteMetadata.siteUrl}/${post.slug}</link>
     <description>${post.summary}</description>
     <pubDate>${new Date(post.date).toUTCString()}</pubDate>
     <author>${siteMetadata.email} (${siteMetadata.author})</author>