Documentation: dynamic-debug-howto: fix example

dynamic debug is "expecting pairs of match-spec <value>" so the example
for all files of which the paths include "usb" there is "file" missing.
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Link: https://lore.kernel.org/r/20210303091646.773111-1-martin.kepplinger@puri.smSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/dynamic-debug-howto.rst

@@ -347,7 +347,7 @@ Examples
 				<debugfs>/dynamic_debug/control
 
   // enable messages in files of which the paths include string "usb"
-  nullarbor:~ # echo -n '*usb* +p' > <debugfs>/dynamic_debug/control
+  nullarbor:~ # echo -n 'file *usb* +p' > <debugfs>/dynamic_debug/control
 
   // enable all messages
   nullarbor:~ # echo -n '+p' > <debugfs>/dynamic_debug/control