Right at the bottom of page 52 of David Tong's QFT notes we have just defined the time ordered Dyson formula, David Tong then shows the expansion of $(3.20)$ however an extra second-order term has appeared. The second-order term in the expansion of the exponential has two terms and the integration limits have been changed from what we see in $(3.18)$.
I can't figure out why this would be the case and this change is introduced with little/no explanation. I imagine it comes from the introduction of the time ordering symbol in some way but exactly how is unclear to me, could someone please briefly explain this?