terminology: crate nrf51 has been renamed to nrf51-pac

See https://docs.rs/nrf51/0.10.0/nrf51/

Author: LeoniePhiline

microbit/src/04-meet-your-hardware/terminology.md

@@ -37,7 +37,7 @@ does actually have a [BSP] so we are going to use that on top of our
 HAL as well.
 
 [nrF52]: https://crates.io/crates/nrf52833-pac
-[nrF51]: https://crates.io/crates/nrf51
+[nrF51]: https://crates.io/crates/nrf51-pac
 [nrF52-hal]: https://crates.io/crates/nrf52833-hal
 [nrF51-hal]: https://crates.io/crates/nrf51-hal
 [BSP]: https://crates.io/crates/microbit