[skip ci] Fix another msh-file bug

...t/poisson/facedir-facemod-variation/generate_meshfiles.py

.../poisson/facedir-facemod-variation/grid_structured_05.msh

.../poisson/facedir-facemod-variation/grid_structured_11.msh

.../poisson/facedir-facemod-variation/grid_structured_17.msh

.../poisson/facedir-facemod-variation/grid_structured_23.msh

.../poisson/facedir-facemod-variation/grid_structured_29.msh

.../poisson/facedir-facemod-variation/grid_structured_30.msh

.../poisson/facedir-facemod-variation/grid_structured_31.msh

.../poisson/facedir-facemod-variation/grid_structured_32.msh

.../poisson/facedir-facemod-variation/grid_structured_33.msh

.../poisson/facedir-facemod-variation/grid_structured_34.msh

.../poisson/facedir-facemod-variation/grid_structured_35.msh

...poisson/facedir-facemod-variation/grid_transformed_05.msh

...poisson/facedir-facemod-variation/grid_transformed_11.msh

...poisson/facedir-facemod-variation/grid_transformed_17.msh

...poisson/facedir-facemod-variation/grid_transformed_23.msh

...poisson/facedir-facemod-variation/grid_transformed_29.msh

...poisson/facedir-facemod-variation/grid_transformed_30.msh

...poisson/facedir-facemod-variation/grid_transformed_31.msh

...poisson/facedir-facemod-variation/grid_transformed_32.msh