Minor edit to mojo/public/cpp/system/README.md

DataPipe's members are producer_handle and consumer_handle, and not producer
and consumer. This is a minor CL to update the references.

TBR=rockot@chromium.org

Change-Id: Ia2dc6686ecd83d5e587a4b33b73599180090b642
Reviewed-on: https://chromium-review.googlesource.com/1150429
Commit-Queue: Helen Li <xunjieli@chromium.org>
Reviewed-by: Helen Li <xunjieli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577993}

mojo/public/cpp/system/README.md

@@ -95,8 +95,8 @@ helpers for more strongly-typed data pipe usage:
 
 ``` cpp
 mojo::DataPipe pipe;
-mojo::ScopedDataPipeProducerHandle producer = std::move(pipe.producer);
-mojo::ScopedDataPipeConsumerHandle consumer = std::move(pipe.consumer);
+mojo::ScopedDataPipeProducerHandle producer = std::move(pipe.producer_handle);
+mojo::ScopedDataPipeConsumerHandle consumer = std::move(pipe.consumer_handle);
 
 // Or alternatively:
 mojo::ScopedDataPipeProducerHandle producer;