Fix the time units stated in the delay and period functon headers
Fix the time units stated in the delay and period functon headers

delay() and period() still stated that the units were milliseconds
whereas in fact they are now a Chrono duration.

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed