Fix comment typo

iwyu_ast_util.cc

@@ -278,7 +278,7 @@ UseFlags ComputeUseFlags(const ASTNode* ast_node) {
 
   // Definitions of free functions are a little special, because they themselves
   // count as uses of all prior declarations (ideally we should probably just
-  // require one but it's hard so say which, so we pick all previously seen).
+  // require one but it's hard to say which, so we pick all previously seen).
   // Later IWYU analysis phases do some canonicalization that isn't
   // necessary/valid for this case, so mark it up for later.
   if (const auto* fd = ast_node->GetAs<FunctionDecl>()) {