Fix typo (def of satisfaction in prop. logic)

[GKerfImf]

As it is explained in the commit message, in the disjunctive case: φ ≡ (ψ ∨ χ), it should be

σ ⊨ φ iff σ ⊨ ψ or σ ⊨ χ, 

and not

σ ⊨ φ iff σ ⊨ φ or σ ⊨ ψ
              ^^

[rzach]

Right you are! Thanks.