Fix setUpdateHandler docstring

[dandavison]

Fixes setUpdateHandler docstring to document that first argument must be a workflow.Context.

How this was tested

I ran this code as the workflow in the update sample in the samples repo, with and without the first argument

func Counter(ctx workflow.Context) (int, error) {
	counter := 0
	err := workflow.SetUpdateHandlerWithOptions(
		ctx,
		"add",
		func(ctx workflow.Context, val int) (int, error) {
			counter += val // note that this mutates workflow state
			return counter, nil
		},
		workflow.UpdateHandlerOptions{
			Validator: func(val int) error {
				if val < 0 { // reject attempts to add negative values
					return fmt.Errorf("invalid addend: %v", val)
				}
				return nil
			},
		})
	if err != nil {
		return 0, err
	}
	_ = ctx.Done().Receive(ctx, nil)
	return counter, nil
}