change unit of txPower from dB to dBm in API.md

[robinfru]

According to line 148 of the LoRa.cpp file, correct the API tx power documentation.

Source : https://github.com/sandeepmistry/arduino-LoRa/blob/47d7f7793246df9444cc996ceb83567145764cbe/src/LoRa.cpp#L148