Update README.md

Fix typo in example

Author: PeeJay

README.md

@@ -722,7 +722,7 @@ public:
   // or the high-power mode above 17 dBm. In other words, it lets the
   // LMIC-determined policy determine what's to be done.
 
-  virutal TxPowerPolicy_t getTxPowerPolicy(
+  virtual TxPowerPolicy_t getTxPowerPolicy(
     TxPowerPolicy_t policy,
     int8_t requestedPower,
     uint32_t frequency