bignum_core.h: Comment update for mbedtls_mpi_core_get_mont_R2_unsafe
bignum_core.h: Comment update for mbedtls_mpi_core_get_mont_R2_unsafe

Signed-off-by: Minos Galanakis <bb9c15d46ed7ecad5455c4c22ac024636068eda6@arm.com>

Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls