In a narrow sense, Trump is right. The publicly released rationale for firing Comey - which was completely sound - came from Rosenstein. And Rosenstein's the one who appointed an independent counsel in the person of Robert Mueller to investigate the whole Russia thing. Once you turn an independent counsel loose, of course, his mission pretty much expands to whatever he wants it to be. So now they're looking into Trump himself on the grounds that he may have obstructed justice by asking Comey to lay off Mike Flynn, and then by subsequently firing him when he didn't.
So yeah: Rosenstein recommended that Trump fired Comey. Trump did it. Now the independent counsel appointed by Rosenstein is investigating Trump for the idea that the firing of Comey was really his punishment for not giving Flynn a pass.
I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt