Update version in tools_intro.md

Add updated GNU Arm Embedded version to this PR.

Author: Amanda Butler

docs/tools/tools_intro.md

@@ -40,7 +40,7 @@ Mbed OS 5 can be built with various toolchains. The currently supported versions
   - A paid version is available as [Arm Compiler 6.13 Professional](https://developer.arm.com/products/software-development-tools/compilers/arm-compiler/downloads/version-6).
   - A paid version is also included in [Keil MDK 5.29](http://www2.keil.com/mdk5/529).
 - [Arm Compiler 5.06 update 6 (to be deprecated in the future)](https://developer.arm.com/products/software-development-tools/compilers/arm-compiler-5/downloads).
-- [GNU Arm Embedded version  6 (6-2017-q1-update)](https://developer.arm.com/open-source/gnu-toolchain/gnu-rm/downloads).
+- [GNU Arm Embedded version 9 (9-2019-q4-major)](https://developer.arm.com/open-source/gnu-toolchain/gnu-rm/downloads).
 - [IAR Embedded Workbench 8.32.1](https://iar.com/mbed).
 
 ## Arm Mbed Studio