By Tzvi Machlin There’s been relatively little good news on Roger Federer as the tennis superstar continues rehab on a slew of career-threatening injuries. But today there was finally a positive update that may give fans a sigh of relief. Taking to Instagram on Monday, the 20-time Grand Slam winner posted a series of photos from his recent rehab training. He declared that his rehab is going great, much to the delight of his 9.1 million IG followers. “Rehab is rockinggggggg,” Federer wrote. The post is only the third that Federer has made on Instagram this year. His last one was a short video o…