Update README.md

Author: kose-y

README.md

@@ -58,8 +58,9 @@ To execute code with MPI calls on all workers, use `@mpi_do`.
 For example:
 ```
 @mpi_do manager begin
+  using MPI
   comm=MPI.COMM_WORLD
-  println("Hello world, I am $(MPI.Comm_rank(comm)) of $(MPI.Comm_size(comm))"))
+  println("Hello world, I am $(MPI.Comm_rank(comm)) of $(MPI.Comm_size(comm))")
 end
 ```
 executes on all MPI workers belonging to `manager` only