docs(github-nvim-theme): update README.md (#1296)

* feat: update github-nvim-theme README.md

* Forgot the original themes without variant names

* Update README.md

---------

Co-authored-by: Uzair Aftab <[email protected]>

Author: pmanole-volvocars

lua/astrocommunity/colorscheme/github-nvim-theme/README.md

@@ -6,12 +6,14 @@ Github's Neovim themes
 
 ## Usage
 
-In your `user/init.lua` file, put:
+In your `plugins/astroui.lua` file, put:
 
 ```lua
 return {
-    ...
-    colorscheme = "github_dark",
+    "AstroNvim/astroui",
+    opts = {
+        colorscheme = "github_dark",
+    }
 }
 ```