function/stdlib: SetSymmetricDifferenceFunc is actually SetSubtractFunc

[apparentlymart]

Due to what I assume was a copy-paste error, the SetSymmetricDifferenceFunc seems to actually be implementing SetSubtractFunc due to calling the wrong method on the underlying set implementation.

go-cty/cty/function/stdlib/set.go

Lines 88 to 105 in f21e35a

	var SetSymmetricDifferenceFunc = function.New(&function.Spec{
	Params: []function.Parameter{
	{
	Name: "first_set",
	Type: cty.Set(cty.DynamicPseudoType),
	AllowDynamicType: true,
	},
	},
	VarParam: &function.Parameter{
	Name: "other_sets",
	Type: cty.Set(cty.DynamicPseudoType),
	AllowDynamicType: true,
	},
	Type: setOperationReturnType,
	Impl: setOperationImpl(func(s1, s2 cty.ValueSet) cty.ValueSet {
	return s1.Subtract(s2)
	}),
	})

There are also no unit tests for either SetSubtractFunc or SetSymmetricDifferenceFunc.