Update README.md

Typo fix in waterfall example

1 files changed, 1 insertions, 1 deletions

diff --git a/README.md b/README.md
index fa3f6f7..e80466e 100644
--- a/README.md
+++ b/README.md
@@ -103,7 +103,7 @@ async.waterfall([
           }
           // since we did not return, this callback still will be called and
           // `processData` will be called twice
-          callback(result);
+          callback(null, result);
         });
     },
     processData