[gardening] Fix recently introduced typo: "declararion" → "declaration" (#2442)

Author: practicalswift

include/swift/AST/Attr.h

@@ -630,7 +630,7 @@ enum class UnconditionalAvailabilityKind {
   /// The declaration is unavailable in the current version of Swift,
   /// but was available in previous Swift versions.
   UnavailableInCurrentSwift,
-  /// The declararion is unavailable for other reasons.
+  /// The declaration is unavailable for other reasons.
   Unavailable,
 };