Docs: usb: update comment and code near increment usage count

update comment: increment our usage count ..
and code according to usb-skeleton.c

Signed-off-by: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Link: https://lore.kernel.org/r/38c6205d6b5c612902f9c187154c05e9cca51107.1635591623.git.philipp.g.hortmann@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/driver-api/usb/writing_usb_driver.rst

@@ -170,8 +170,8 @@ structure. This is done so that future calls to file operations will
 enable the driver to determine which device the user is addressing. All
 of this is done with the following code::
 
-    /* increment our usage count for the module */
-    ++skel->open_count;
+    /* increment our usage count for the device */
+    kref_get(&dev->kref);
 
     /* save our object in the file's private structure */
     file->private_data = dev;