Add trailing slash to site_url

`site_url` has to end with a trailing slash, because when MkDocs processes the Jinja2 templates, it will embed a script tag into the final html:
```js
<script id="__config" type="application/json">{"base": "/some/path/4.1.1", ...}</script>
```

The JS-relevant part of mike then handles this as follows
`fetch(ABS_BASE_URL + "../versions.json")`
or, if you look at the actual js-bundle:
`new URL("../versions.json", t.base)`

And if `t.base` doesn't end with a trailing slash, the last component is completely ignored, which is obviously not what we want.

Author: cubernetes

mike/mkdocs_plugin.py

@@ -46,7 +46,7 @@ def on_config(self, config):
         if version and config.get('site_url'):
             if self.config['canonical_version'] is not None:
                 version = self.config['canonical_version']
-            config['site_url'] = urljoin(config['site_url'], version)
+            config['site_url'] = urljoin(config['site_url'], version) + '/'
 
     def on_files(self, files, config):
         if not self.config['version_selector']: