Update spec/08-pattern-matching.md

Co-authored-by: Fengyun Liu <liu@fengy.me>

spec/08-pattern-matching.md

@@ -240,7 +240,7 @@ the pattern.
 An extractor pattern cannot match the value `null`. The implementation
 ensures that the `unapply`/`unapplySeq` method is not applied to `null`.
 
-The type `O` is said to be an _extractor type_ for some type `T` if it has a
+A type is said to be an _extractor type_ for some type `T` if it has a
 method `get` with return type `T`, and a method `isEmpty` with a return type
 that conforms to `Boolean`. `Option[T]` is an extractor type for type `T`.