Add missing **/ to core_or.dox

/**

@dir /core/or

@brief core/or: *Onion routing happens here*.

@dir core/or

@brief core/or: **Onion routing happens here!**

This is the central part of Tor that handles the core tasks of onion routing:

building circuit, handling circuits, attaching circuit to streams, moving

`scheduler.c`

: Decides which channel/circuit pair is ready to receive the next cell.

**/