updated README to ChibiOS ver_2.6.8 commit

README.txt

@@ -112,10 +112,10 @@ install the according 32-bit versions of the packages:
 Since AMiRo-OS uses ChibiOS as underlying system kernel, you need to acquire a
 copy of it as well. First, go to the directory which contains the AMiRo-OS
 folder (but do not go into the AMiRo-OS directory itself!). Now clone the GIT
-repository of ChibiOS and checkout version 2.6.x:
+repository of ChibiOS and checkout version 2.6.8:
   >$ git clone https://github.com/ChibiOS/ChibiOS.git ChibiOS
   >$ cd ChibiOS
-  >$ git checkout 2e6dfc7364e7551483922ea6afbaea8f3501ab0e
+  >$ git checkout ver_2.6.8 
 It is highly recommended to use exactly this commit. Although newer commits in
 the 2.6.x branch might work fine, AMiRo-OS is not compatible with ChibiOS
 version 3 or newer.