[fix] Gras-Sim compute_N_plant_litter
Summary
missing parenthesis in https://gitlab.uliege.be/deal-public/pase/-/blame/develop/MODULES/CROPS/GRASSIM/run_grassim.py?ref_type=heads#L68
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missing parenthesis in https://gitlab.uliege.be/deal-public/pase/-/blame/develop/MODULES/CROPS/GRASSIM/run_grassim.py?ref_type=heads#L68