Martha Stewart took a relaxing dip in a hot tub during her 85th birthday getaway to Maine — and her fans are obsessed with her sultry look. The lifestyle mogul was pictured in a black spaghetti strap swimsuit in the jetted spa, which featured a forest backdrop and was situated alongside an expansive pool. Stewart wore a neutral makeup palette and styled her hair in a soft flip while giving the camera a sultry expression as she enjoyed the heated water. “I am going to use this amazing hot tub every single day I am in maine,” she captioned the snapshot on Tuesday. “Invigorating restorative Therapeutic relaxing.” In the comments thread,…