Correct documentation of mbedtls_platform_gmtime_r()
Correct documentation of mbedtls_platform_gmtime_r()

Previous documentation stated that gmtime_r() was from the standard library,
but it's POSIX.

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