Update usbd-serial to match.

Author: qwandor

examples/usb/Cargo.toml

@@ -6,10 +6,10 @@ edition = "2018"
 publish = false
 
 [dependencies]
-cortex-m-rt = "0.7.0"
+cortex-m-rt = "0.7.4"
 panic-semihosting = "0.6.0"
 usb-device = "0.3.2"
-usbd-serial = "0.1.0"
+usbd-serial = "0.2.1"
 
 [dependencies.nrf52840-hal]
 path = "../../nrf52840-hal"