Fix MPI versions in MPI.3 manpage

Thanks to Andy Riebs for reporting that on the Open MPI user mailing list (https://www.mail-archive.com/users@lists.open-mpi.org/msg34103.html)

Signed-off-by: Joseph Schuchart <schuchart@hlrs.de>

ompi/mpi/man/man5/Open-MPI.5.md

@@ -16,8 +16,8 @@ message-passing programs that would be efficient, flexible, and
 portable.
 
 The outcome, known as the MPI Standard, was first published in 1993; its
-most recent version (MPI-2) was published in July 1997. Open MPI 1.2
-includes all MPI 1.2-compliant and MPI 2-compliant routines.
+most recent version (MPI-3.1) was published in June 2015. Open MPI
+includes all MPI 3.1-compliant routines.
 
 For more information about Open MPI, see
 [https://www.open-mpi.org](https://www.open-mpi.org).