docs(gitblame): Use setup (#421)

There was an [issue](f-person/git-blame.nvim#65)
on git-blame.nvim about `vim.g.gitblame_disable = 0` not working with
Lunar.

I recently implemented a setup function which solves this issue.
This commit uses `setup`, instead of legacy vim.g in order to avoid any
confusions for users who wanna use git-blame :).

versioned_docs/version-1.3/configuration/plugins/example-configurations.md

@@ -312,7 +312,7 @@ vim.keymap.set('n', ',W', swap_windows, { desc = 'Swap windows' })
   event = "BufRead",
   config = function()
     vim.cmd "highlight default link gitblame SpecialComment"
-    vim.g.gitblame_enabled = 0
+    require("gitblame").setup { enabled = false }
   end,
 },
 ```