fix readme

Author: huanchenz

README.md

@@ -17,8 +17,10 @@ We also host a [demo website](https://www.rangefilter.io/).
 
 ## Simple Example
 A simple example can be found [here](https://github.com/efficient/SuRF/blob/master/simple_example.cpp). To run the example:
-  g++ -mpopcnt -std=c++11 simple_example.cpp
-  ./a.out
+```
+g++ -mpopcnt -std=c++11 simple_example.cpp
+./a.out
+```
 
 ## Run Unit Tests
     make test