Documentation: dynamic_debug: fix a couple of typos

Fix a few "typos" in dynamic-debug-howto.rst.

s/dyndbg_query/ddebug_query/
s/sysfs/debugfs/

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jason Baron <jbaron@akamai.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>