Update guaranteed_income.md
Update guaranteed_income.md

Changed text about when you must buy an annuity (previously said within 6 months) because ABI told us this was incorrect - Users have 6 months but providers are unlikely to release the tax-free cash until they know what you're doing with the rest.
guidance-guarantee-programme/pension_guidance,guidance-guarantee-programme/pension_guidance,guidance-guarantee-programme/pension_guidance