Correct function name in README.md

The README references a `publish_batch` function.
I couldn't get that to work; digging through the source, it looks like it intends to refer to the `batch_publish` function:

https://github.com/nicholassm/disruptor-rs/blob/22e13239eddf5f6695238dbde8d6121059dfeb69/src/producer.rs#L218

README.md

@@ -50,7 +50,7 @@ fn main() {
     }
 
     // Publish a batch of events into the Disruptor.
-    producer.publish_batch(5, |iter| {
+    producer.batch_publish(5, |iter| {
         for e in iter { // `iter` is guaranteed to yield 5 events.
             e.price = 42.0;
         }