docs: typo neovim -> my-neovim

When I tried this code I was getting an error `attribute 'neovim' missing`. When i changed it to `my-neovim` (from the other code above), it worked.

docs/manual/installation/standalone/nixos.md

@@ -52,7 +52,7 @@ the default theme enabled. You may use other options inside `config.vim` in
           # Can also move this to another config file if you pass your own
           # inputs/self around with specialArgs
           ({pkgs, ...}: {
-            environment.systemPackages = [self.packages.${pkgs.stdenv.system}.neovim];
+            environment.systemPackages = [self.packages.${pkgs.stdenv.system}.my-neovim];
           })
         ];
         # ...