Manual includes non-existant ExitPolicyDefault option

Back in November I made a few manual fixes which included making its ExitPolicyDefault line visible...

https://gitweb.torproject.org/tor.git/commit/?id=3563a2c

On reflection the tor codebase doesn't look to include any such option. If it wasn't for a torrc option then I'm unsure what ExitPolicyDefault was in reference to...

Most likely we should just drop it from the manual.