docs: simplify 'cd'ing into TF-M folder

Currently docs use two 'cd' commands to move to the TF-M
folder (1. move to base folder 2. move to TF-M folder). This patch
simplifies to a single 'cd' command, moving directly to TF-M base
folder.

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
Change-Id: I8d67b73d749962672aca4b351ae5b18efb38283e
2 files changed