usb: gadget: drop superfluous ':' in doc string

There was one superfluous ':' that kernel-doc complained about.
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Closes: https://lore.kernel.org/all/c718a490-028d-2682-9ad7-8256d16504bf@infradead.org/
Fixes: fb6211f1 ("usb: gadget: add doc to struct usb_composite_dev")
Signed-off-by: Jó Ágila Bitsch <jgilab@gmail.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/ZEQFzMntIrwvZl4+@jo-einhundertSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/usb/composite.h

@@ -443,7 +443,7 @@ static inline struct usb_composite_driver *to_cdriver(
  * @bcd_webusb_version: 0x0100 by default, WebUSB specification version
  * @b_webusb_vendor_code: 0x0 by default, vendor code for WebUSB
  * @landing_page: empty by default, landing page to announce in WebUSB
- * @use_webusb:: false by default, interested gadgets set it
+ * @use_webusb: false by default, interested gadgets set it
  * @os_desc_config: the configuration to be used with OS descriptors
  * @setup_pending: true when setup request is queued but not completed
  * @os_desc_pending: true when os_desc request is queued but not completed