docs: Update missing python3-pexpect as build prerequisites

Javier Tiarequested to merge
TRS-391 into main

In the most recent Poky version, python3-pexpect is required.

In the future, the apt package list could be out-dated; mention checking the Yocto documentation to validate the required packages.

Signed-off-by: Javier Tia javier.tia@linaro.org

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