[gardening] Fix a double "the" in a comment

Author: hamishknight

lib/AST/ASTVerifier.cpp

@@ -2734,8 +2734,8 @@ class Verifier : public ASTWalker {
       const DeclContext *DC = GTPD->getDeclContext();
 
       // Skip verification of deserialized generic param decls that have the
-      // the file set as their parent. This happens when they have not yet had
-      // their correct parent set.
+      // file set as their parent. This happens when they have not yet had their
+      // correct parent set.
       // FIXME: This is a hack to workaround the fact that we don't necessarily
       // parent a GenericTypeParamDecl if we just deserialize its type.
       if (auto *fileDC = dyn_cast<FileUnit>(DC)) {