Downgrade cortex-m-rt in example.

0.7.4 was yanked.

Author: qwandor

examples/usb/Cargo.toml

@@ -6,7 +6,7 @@ edition = "2018"
 publish = false
 
 [dependencies]
-cortex-m-rt = "0.7.4"
+cortex-m-rt = "0.7.3"
 panic-semihosting = "0.6.0"
 usb-device = "0.3.2"
 usbd-serial = "0.2.1"