Update README.md

Author: RunzhouHan

README.md

@@ -100,9 +100,9 @@ PROV-IO Syscall wrapper is developed to track high frequency POSIX I/O APIs such
 Syscall Wrapper is still under testing, stay tuned!
 
 ## User Engine
-## Query Engine
+### Query Engine
 Check out query engine at: https://github.com/hpc-io/prov-io/tree/master/user_engine/query.
-## Visualization
+### Visualization
 Check out visualizer at: https://github.com/hpc-io/prov-io/tree/master/user_engine/visualizer. An example of visualized RDF provenance is also given.