Fix USB docs (#153)

src/usb.rs

@@ -1,8 +1,8 @@
 //! USB peripheral
-
+//!
 //! Requires the `stm32-usbd` feature.
 //! See https://github.com/stm32-rs/stm32f1xx-hal/tree/master/examples
-//! for usage examples
+//! for usage examples.
 
 use crate::pac::{RCC, USB};
 use stm32_usbd::UsbPeripheral;