Fix typo in USERS-GUIDE.md
Fix typo.
# DAPLink users guide
-DAPLink provides three interface. These are drag-and-drop programming, a serial port and debugging support. In addition, you can update DAPLink firmware using the drag-and-drop programming interface of the bootloader.
+DAPLink provides three interfaces. These are drag-and-drop programming, a serial port and debugging support. In addition, you can update DAPLink firmware using the drag-and-drop programming interface of the bootloader.
## Drag-and-drop programming
