Fix missing word.

Co-authored-by: Ben Rimmington <[email protected]>

Author: amartini51

stdlib/public/core/Optional.swift

@@ -10,7 +10,7 @@
 //
 //===----------------------------------------------------------------------===//
 
-/// A type that represents either a wrapped value the absence of a value.
+/// A type that represents either a wrapped value or the absence of a value.
 ///
 /// You use the `Optional` type whenever you use optional values, even if you
 /// never type the word `Optional`. Swift's type system usually shows the