fix(disable_tabline): fix recipe to remove any non-default showtabline setting

Author: mehalter

src/content/docs/recipes/disable_tabline.mdx

@@ -44,6 +44,9 @@ return {
     "AstroNvim/astrocore",
     ---@param opts AstroCoreOpts
     opts = function(_, opts)
+      if vim.tbl_get(opts, "options", "opt", "showtabline") then
+        opts.options.opt.showtabline = nil
+      end
       for k, _ in pairs(opts.mappings.n) do
         if k:find("^<Leader>b") then
           opts.mappings.n[k] = false