Fix rust-lang#2625

"Run this code" should be "compile this code" (ch18-03-pattern-syntax)

Fixes rust-lang#2625

src/ch18-03-pattern-syntax.md

@@ -344,7 +344,7 @@ warning because that could be a bug. But sometimes it’s useful to create a
 variable you won’t use yet, such as when you’re prototyping or just starting a
 project. In this situation, you can tell Rust not to warn you about the unused
 variable by starting the name of the variable with an underscore. In Listing
-18-20, we create two unused variables, but when we run this code, we should
+18-20, we create two unused variables, but when we compile this code, we should
 only get a warning about one of them.
 
 <span class="filename">Filename: src/main.rs</span>