doc: remove BUILD_ALWAYS True from sysbuild documentation

The examples showing how to include an extra Zephyr project into the
build wrongly used `BUILD_ALWAYS True`.

The `ExternalZephyrProject_Add()` already adds this flag implicit to
ensure Zephyr based projects are always built correctly.

Thus remove the flag in the doc example as it is wrong and causes CMake
error if users tries to pass on this flag.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>