Update README.md

Author: fpistm

README.md

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 # Arduino core support for STM32 based boards 
 [![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/STM32-2017.8.4.svg)](https://github.com/stm32duino/Arduino_Core_STM32/compare/STM32-2017.8.4...master)
+[![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)
 
 ## Boards available: