Update README.md

Author: fpistm

README.md

@@ -1,6 +1,6 @@
 # Arduino core support for STM32 based boards
 ### Latest release [![GitHub release](https://img.shields.io/github/release/stm32duino/Arduino_Core_STM32.svg)](https://github.com/stm32duino/Arduino_Core_STM32/releases/latest)
-[![GitHub commits](https://img.shields.io/github/commits-since/stm32duino/Arduino_Core_STM32/2017.8.31.svg)](https://github.com/stm32duino/Arduino_Core_STM32/compare/2017.8.31...master)
+[![GitHub commits](https://img.shields.io/github/commits-since/stm32duino/Arduino_Core_STM32/2017.9.22.svg)](https://github.com/stm32duino/Arduino_Core_STM32/compare/2017.9.22...master)
 
 * [Getting Started](https://github.com/stm32duino/Arduino_Core_STM32#getting-started)<br>
 * [Boards available](https://github.com/stm32duino/Arduino_Core_STM32#boards-available)<br>