doc: ti: j7200_evm: Fix OPTEE platform name

k3-j7200 does not exist in upstream OPTEE. Use j721e as the platform
name. Using k3-j7200 as OPTEE name results in broken boot due to wrong
configuration being picked.

Fixes: c727b81d ("doc: board: ti: k3: Reuse build instructions")
Signed-off-by: Nishanth Menon <nm@ti.com>