[fix] Multisite issue when using `find` function

[hdwebpros]

When running multisite with subfolder, the route includes the subfolder. The find function returns null since it only needs the page route, without the subfolder.

This commit removes that subfolder as $this->base, which fixes the route on multisite with subfolder.

This has been tested on multisite with Grav v1.7.30 and Flex Objects