USB: Documentation: Use correct character in gadget_hid.txt

Use ASCII minus sign instead of unicode EN DASH in gadget_hid.txt. Also remove
two unecessary spaces in the same file.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>