Add quotes to the default pinned-tab-marker

It's a fix for: https://todo.sr.ht/~sircmpwn/aerc2/361
Using an unquoted backtick char is leading to a config error:

```
Failed to load config: missing closing key quote from '`
' to ''
```

config/aerc.conf.in

@@ -46,7 +46,7 @@ new-message-bell=true
 # Marker to show before a pinned tab's name.
 #
 # Default: `
-pinned-tab-marker=`
+pinned-tab-marker='`'
 
 # Describes the format string to use for the directory list
 #