Fix CPU frequency env var in config.toml

Fixes #41

Author: rockboynton

.cargo/config.toml

@@ -6,5 +6,4 @@ build-std = ["core"]
 
 # Cargo versions before 2021-02-23 won't recognize this: https://github.com/rust-lang/cargo/pull/9175
 [env]
-## AVR_CPU_FREQUENCY_HZ = 16000000
-## because https://github.com/avr-rust/blink/issues/41
+AVR_CPU_FREQUENCY_HZ = "16_000_000"