Update README.md

Author: sh123

README.md

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 # Introduction
-Experimental Amateur Radio single sketch Arduino NRF24L01 2.4 GHz (23cm band) USB serial KISS TNC/modem. 
+Experimental Amateur Radio single sketch Arduino NRF24L01 2.4 GHz (13cm band) USB serial KISS TNC/modem. 
 
 Could be used with Linux AX25 tools to setup packet network or other tools such as [APRSDroid](https://aprsdroid.org) or [Codec2 Walkie-Talkie](https://github.com/sh123/codec2_talkie).