Update CMSIS headers fetching instructions

Clarified instructions for fetching CMSIS headers.

Author: felipe-iar

NOTES.md

@@ -7,7 +7,7 @@ Some of the example projects use legacy versions of CMSIS. If when building a pr
 
 >Fatal Error[Pe1696]: cannot open source file “core_cm3.h”
 
-The recommended solution is to use `git` for fetching up-to-date CMSIS headers:
+The recommended solution is to use `git` for fetching the required CMSIS headers:
 * Linux (as root):
 ```
 cd /opt/iar/