Use a more coincise symbol to signal compilation errors in slides

Author: LukeMathWalker

src/idiomatic/leveraging-the-type-system/newtype-pattern.md

@@ -19,17 +19,15 @@ fn double(n: u64) -> u64 {
     n * 2
 }
 
-// This doesn't compile ❌
-double(UserId(1));
+double(UserId(1)); // 🛠️❌
 ```
 
 The Rust compiler won't implicitly convert to (or from) the underlying type.\
 It won't let you use methods or operators defined on the underlying type either:
 
 ```rust,compile_fail
 # pub struct UserId(u64);
-// This doesn't compile ❌
-assert_ne!(UserId(1), UserId(2));
+assert_ne!(UserId(1), UserId(2)); // 🛠️❌
 ```
 
 <details>

src/idiomatic/leveraging-the-type-system/newtype-pattern/semantic-confusion.md

@@ -35,8 +35,7 @@ pub fn login(username: &Username, password: &Password) -> Result<(), LoginError>
 
 # let password = Password("password".into());
 # let username = Username("username".into());
-// Compiler error 🎉
-login(password, username);
+login(password, username); // 🛠️❌
 ```
 
 <details>