chore(breadcrumbs): remove `Markdown` from excluded file types (#3196)

The output of `vim.bo.filetype` for Markdown files is `markdown` not
`Markdown` so this entry did nothing.

lua/lvim/core/breadcrumbs.lua

@@ -29,7 +29,6 @@ M.config = function()
       "dap-terminal",
       "dapui_console",
       "lab",
-      "Markdown",
       "notify",
       "noice",
       "",