docs: fix dev_debug() braino in dynamic-debug-howto.txt

While looking at dynamic-debug-howto.txt, I noticed that it references
dev_debug() (which doesn't exist) instead of dev_dbg() (which does
exist).  I'm lazy, so I choose to fix the document rather than changing
every dev_dbg() call.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>