README.md: fix syntax error in example lazy.nvim plugin installation

[cdunham]

NVIM v0.11.4 (via neovim-snap on Ubuntu 24.04) and the latest lazy.nvim gives the error "unexpected symbol near '{'" without the return keyword.

[Julian]

Whether you need return or not depends on whether you take the example there and insert it alongside existing plugins you use or whether you put it in an entirely new plug-in file specifically for lean.nvim.

[cdunham]

Makes sense, thanks.