docs(measured-boot): update the build command

As per recent changes to OPTEE's fvp.mk file, both options
"MEASURED_BOOT" and "MEASURED_BOOT_FTPM" are required for the fTPM
application to be built.

Change-Id: I621113c3fbd47e9f5be015ea65e9b8d0f218e4e8
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
1 file changed