Fix rtic example build by updating to newer pre-release

Author: Sh3Rm4n

Cargo.toml

@@ -59,8 +59,8 @@ panic-probe = "0.2.0"
 panic-semihosting = "0.5.6"
 usbd-serial = "0.1.1"
 usb-device = "0.2.8"
-cortex-m-rtic = "=0.6.0-alpha.5"
-dwt-systick-monotonic = "=0.1.0-alpha.3"
+cortex-m-rtic = "=0.6.0-rc.2"
+dwt-systick-monotonic = "=0.1.0-rc.1"
 panic-rtt-target = { version = "0.1", features = ["cortex-m"] }
 rtt-target = { version = "0.3.0", features = ["cortex-m"] }