man: MPI_Get_version now returns 3.1 instead of 2.1.

ompi/mpi/man/man3/MPI_Get_version.3in

@@ -55,7 +55,7 @@ Fortran only: Error status (integer).
 
 .SH DESCRIPTION
 .ft R
-Since Open MPI is MPI 2.1 compliant, this function will return a version value of 2 and a subversion value of 1 for this release.
+Since Open MPI is MPI 3.1 compliant, this function will return a version value of 3 and a subversion value of 1 for this release.
 
 .SH NOTE
 .ft R