Minor corrections

Irrefutable pattern receives warning, refutable error, when used unexpectedly.

src/ch18-02-refutability.md

@@ -61,7 +61,7 @@ the code in the curly brackets, giving it a way to continue validly. Listing
 patterns instead of `let`</span>
 
 We’ve given the code an out! This code is perfectly valid, although it means we
-cannot use an irrefutable pattern without receiving an error. If we give `if
+cannot use an irrefutable pattern without receiving a warning. If we give `if
 let` a pattern that will always match, such as `x`, as shown in Listing 18-10,
 the compiler will give a warning.
 
@@ -83,7 +83,7 @@ For this reason, match arms must use refutable patterns, except for the last
 arm, which should match any remaining values with an irrefutable pattern. Rust
 allows us to use an irrefutable pattern in a `match` with only one arm, but
 this syntax isn’t particularly useful and could be replaced with a simpler
-`let` statement.
+`if let` statement.
 
 Now that you know where to use patterns and the difference between refutable
 and irrefutable patterns, let’s cover all the syntax we can use to create