README: fix compilation example

Signed-off-by: Karol Mroz <mroz.karol@gmail.com>

README

@@ -483,8 +483,9 @@ MPI Functionality and Features
   your appliation via (e.g., via -lompitrace) will automatically
   output to stderr when some MPI functions are invoked:
 
-  shell$ mpicc hello_world.c -o hello_world -lompitrace
-  shell$ mpirun -np 1 hello_world.c
+  shell$ cd examples/
+  shell$ mpicc hello_c.c -o hello_c -lompitrace
+  shell$ mpirun -np 1 hello_c
   MPI_INIT: argc 1
   Hello, world, I am 0 of 1
   MPI_BARRIER[0]: comm MPI_COMM_WORLD