chore: set scrolloff = 999 to center the active line while editing markdown files

.config/nvim/lua/core/autocmds.lua

@@ -12,6 +12,7 @@ autocmd('FileType', {
       o.breakindent = true
       o.textwidth = 80
       o.spell = true
+      o.scrolloff = 999 -- center active line
    end,
 })