"The jedi master" Suggests that Yoda was the only person who taught him. That would be a lie, regardless of Qui-Gon being dead.
"A Jedi" suggests that Yoda was one of the Jedi that taught him.
It's such an easy thing to fix. Which IMO it should. It's not like I'm gonna lose sleep over it, but Obi-Wan tells enough white lies to Luke in ANH. No need to add more.