docs(README): specify name field in lazy.nvim installation

[S1M0N38]

If you try to install this colorscheme using lazy.nvim in the following way

{
  "projekt0n/github-nvim-theme",
  opts = {
    options = {
      transparent = true,
    },
  }
}

i.e., specifying some options in the opts field instead of using config, lazy cannot find the plugin.

To automatically call setup with the values of opts, it has to infer the name of the plugin from projekt0n/github-nvim-theme, resulting in github-nvim-theme. However, the plugin must be required with require('github-theme').

You can manually specify the name of the plugin using the name field:

{
  "projekt0n/github-nvim-theme",
  name = "github-theme",
  opts = {
    options = {
      transparent = true,
    },
  }
}