Check here, section " I’m still running on nvidia-450 and I want to recompile the 450 driver (Kernel 5.4 LTS):", by clicking on the ► on the line below.
If you’re running linux kernel 5.4 (which you should if you don’t want to get in over your head, because it’s the only LTS kernel on Manjaro as of now), you just have to change any occurrence of “450” with “455” 