doc: fix typo in isl_val_cmp_si documentation
doc: fix typo in isl_val_cmp_si documentation

Signed-off-by: Sven Verdoolaege <235c10dd23b819f81cdc9756a251746bc184cab6@gmail.com>

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