That sounds like a typo, but it actually isn't! You are in the Octoprint directory, which has a venv subdirectory. The previous step implies that you are in the Octoprint directory:
in the source folder:
The period/dot in that case means "from this current folder". It's a common shorthand to tell the shell you are going to be executing a local command. In this case the command is
python, which is in the
venv/bin subdirectories. Plus, if you hadn't done a "cd octoprint",
../venv would be the wrong direction, as that is headed towards a parent folder.
The download page instructions are very approximate- or "generic" as it says. If you click through to the linux/raspbian install instructions, it gives a more complete step by step set of instructions.