Remove unnecessary white line

Summary:
Seems to be causing rendering error:

{F114885894}

Created from Diffusion's 'Open in Editor' feature.

Reviewed By: lskuff

Differential Revision: D6358188

fbshipit-source-id: 3c437007b425bcadb79a71807ad92d520a70e8cb

folly/io/async/README.md

@@ -17,7 +17,6 @@ EventBase base;
 auto thread = std::thread([&](){
   base.loopForever();
 });
-
 ```
 
 EventBase has built-in support for message passing between threads.