A few Freeze documentation buglets

In the Freeze chapter, the examples show inheritance from IceUtil::AbstractMutex, which can't happen. Also, the lock examples still use "Lock lock(*this);". The demo apps all work, inheriting from IceUtil::RWRecMutex and using RLock/WLock as appropriate. I figure these were just oversights from earlier versions of the code

These issues have been fixed for the next release.

Thanks!
- Mark