The par-4 18th hole at Doral has historically played as one of the hardest holes in PGA Tour history.  Jordan Spieth agrees, but he exited the green breathing a sigh of relief.  “Getting a birdie there (just the sixth of the tournament on No. 18 through two days) felt like a huge steal,” Spieth said after a second-round one-under 71. “Just wanted to hit the putt hard enough. They were pretty slow and bumpy this afternoon.” That birdie got the three-time major champion to T2 with Alex Smalley and Nick Taylor at eight-under par after 36 holes of the Cadillac Championship, a signature event, five back of leader Cam Young.  Perhaps the reason Spieth isn’t closer to the lead is because of his putter, which has been the story of the past few years. The 32-year-old is second in the 72-player field in strokes-gained total and fourth in approach, but 48th in putting, losing .816 strokes.  Things shifted a bit late in the round, though.…