Fix AN4759 link

Author: Svein Kristian Rougseth

src/rtc.rs

@@ -1,6 +1,6 @@
 //! Interface to the real time clock. See STM32F303 reference manual, section 27.
 //! For more details, see
-//! [ST AN4759](https:/www.st.com%2Fresource%2Fen%2Fapplication_note%2Fdm00226326-using-the-hardware-realtime-clock-rtc-and-the-tamper-management-unit-tamp-with-stm32-microcontrollers-stmicroelectronics.pdf&usg=AOvVaw3PzvL2TfYtwS32fw-Uv37h)
+//! [ST AN4759](https://www.st.com/resource/en/application_note/an4759-using-the-hardware-realtime-clock-rtc-and-the-tamper-management-unit-tamp-with-stm32-microcontrollers-stmicroelectronics.pdf)
 
 use crate::pac::rtc::{dr, tr};
 use crate::pac::{PWR, RCC, RTC};