doc/CMake: Modify CMake SCP_FIRMWARE_SOURCE_DIR set command example

The provided command in cmake_readme.md that sets the
SCP_FIRMWARE_SOURCE_DIR cache variable introduces errors when
using relative paths.

This patch modifies this command so that relative paths usage
won't introduce errors.

Signed-off-by: Ahmed Gadallah <ahmed.gadallah@arm.com>
Change-Id: Idc2c88d8fc687d6e90bf2e739a68b2cc163f9708