Fix a bug in mpi/schur_pressure_correction.hpp
Fix a bug in mpi/schur_pressure_correction.hpp

Introduced in decfc9e8c1b5e3a7e6be061a229618464a168e66

ddemidov/amgcl,ddemidov/amgcl,ddemidov/amgcl,ddemidov/amgcl