isl_reordering: update documentation
isl_reordering: update documentation

This was missing from isl-0.19-133-g87365a3baa (isl_reordering:
rename "dim" field to "space", Tue Apr 10 12:55:36 2018 +0200).

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl