Fix typo

Author: jamesmunns

book/src/tutorial/104-sdk.md

@@ -5,7 +5,7 @@ For this next section, we'll use the SDK library, `poststation-sdk`. The source
 
 Make sure that:
 
-1. You have installed Rustm using the instructions at [rustup.rs](https://rustup.rs/)
+1. You have installed Rust using the instructions at [rustup.rs](https://rustup.rs/)
 2. Your poststation server **is still running** with a simulator device, on the current machine
 
 ## Create a new binary application project