incorrect docstring for `has_preimage_with_preimage`

[matthias314]

According to the docstring for has_preimage_with_preimage, the second component of the return value is the identity of the domain in case there is no preimage. However, it is a vector:

julia> M = FreeModule(ZZ, 1);

julia> f = ModuleHomomorphism(M, M, matrix(ZZ[2;;]));

julia> b, x = has_preimage_with_preimage(f, only(gens(M)))
(false, (0))

julia> typeof(x)
AbstractAlgebra.Generic.FreeModuleElem{BigInt}

In fact, it's the zero vector of the domain:

AbstractAlgebra.jl/src/ModuleHomomorphism.jl

Lines 161 to 165 in e440963

	if fl
	return true, elem_type(D)[D(x[i:i, 1:m]) for i in 1:length(v)]
	else
	return false, elem_type(D)[D(zero_matrix(R, 1, m)) for x in v]
	end