Add /blog suffix to blog base URL in RSS generator

[flaksp]

Currently, the RSS feed is generated with all links broken because of missing /blog suffix in base variable.

I have marked all broken links:

<?xml version="1.0" encoding="UTF-8" ?>
<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
  <channel>
    <title>JSON Schema Blog RSS Feed</title>
    <link>https://json-schema.org/rss.xml</link> <!-- 👈 -->
    <atom:link rel="self" href="https://json-schema.org/rss.xml" type="application/rss+xml"/> <!-- 👈 -->
    <description>JSON Schema Blog</description>
    <language>en-gb</language>
    <copyright>Made with :love: by the JSON Schema team.</copyright>
    <pubDate>Thu, 01 Sep 2022 09:34:32 GMT</pubDate>
    <generator>next.js</generator>
    <item>
      <title>Using Dynamic References to Support Generic Types</title>
      <description>A step in the right direction for modelling data with JSON Schema</description>
      <link>https://json-schema.org/posts/dynamicref-and-generics?utm_source=rss</link> <!-- 👈 -->
      <category>blog</category>
      <guid isPermaLink="true">https://json-schema.org/posts/dynamicref-and-generics?utm_source=rss</guid> <!-- 👈 -->
      <pubDate>Wed, 31 Aug 2022 00:00:00 GMT</pubDate>
      <enclosure url="https://json-schema.org/img/posts/2022/dynamicref-and-generics/cover.webp" length="15026" type="image/webp"/> <!-- 👈 -->
    </item>
    <!-- ... -->
  </channel>
</rss>

[Relequestual]

Yikes, that's quite broken. If I still used an RSS reader, I might have picked that up. Thank you @flaksp !