tee: optee: fix description in Kconfig

Fix OPTEE_TA_AVB symbol description in Kconfig:
s/"write"rb"/"write_rb"/g

Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>