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>

Documentation/dynamic-debug-howto.txt

@@ -6,7 +6,7 @@ This document describes how to use the dynamic debug (ddebug) feature.
 
 Dynamic debug is designed to allow you to dynamically enable/disable kernel
 code to obtain additional kernel information. Currently, if
-CONFIG_DYNAMIC_DEBUG is set, then all pr_debug()/dev_debug() calls can be
+CONFIG_DYNAMIC_DEBUG is set, then all pr_debug()/dev_dbg() calls can be
 dynamically enabled per-callsite.
 
 Dynamic debug has even more useful features:
@@ -26,7 +26,7 @@ Dynamic debug has even more useful features:
 Controlling dynamic debug Behaviour
 ===================================
 
-The behaviour of pr_debug()/dev_debug()s are controlled via writing to a
+The behaviour of pr_debug()/dev_dbg()s are controlled via writing to a
 control file in the 'debugfs' filesystem. Thus, you must first mount the debugfs
 filesystem, in order to make use of this feature. Subsequently, we refer to the
 control file as: <debugfs>/dynamic_debug/control. For example, if you want to