[Sound-open-firmware] [PATCH] README: correct the root-dir, which didn't work actually

[Liam Girdwood]

On Fri, 2017-12-08 at 23:27 +0800, Keyon Jie wrote:
> Toolchain(xtensa-root) should be installed in the same layer with
> the SOF repo, but maybe not in ~/source/reef/ folder.
> 
> Here correct it by using relative path.
> 
> Signed-off-by: Keyon Jie <yang.jie at linux.intel.com>
> 
Applied.