gofmt -w errors.go (#179)

diff --git a/errors.go b/errors.go
index 1963d86..7421f32 100644
--- a/errors.go
+++ b/errors.go
@@ -229,7 +229,7 @@
 	}
 	return &withMessage{
 		cause: err,
-		msg: fmt.Sprintf(format, args...),
+		msg:   fmt.Sprintf(format, args...),
 	}
 }