fix(propositional_logic_in_lean.rst): fill in missing proof
fix(propositional_logic_in_lean.rst): fill in missing proof

rlewis1988/logic_and_proof,avigad/logic_and_proof,avigad/logic_and_proof,leanprover/logic_and_proof,rlewis1988/logic_and_proof,leanprover/logic_and_proof,avigad/logic_and_proof,rlewis1988/logic_and_proof,leanprover/logic_and_proof