Switched the Neovim and Vim install instructions around.

This is as:
- If someone was installing copilot on Neovim, they would reasonably come across the vim section, follow the instructions and then realise, as they scroll down, they did it incorrectly,
- On the other hand, if someone was installing it on vim, they would be much less likely to read the Neovim part and believe it was for them.

To further emphasise the difference between the sections, as they are hard to tell apart, I put vim and neovim in italics in the subheadings.