fix(ftplugin): correct indent to 4 space

Author: charliie-dev

after/ftplugin/markdown.lua

@@ -3,6 +3,6 @@ local set = vim.opt_local
 set.wrap = true
 set.cindent = true
 set.expandtab = true
-set.shiftwidth = 2
-set.softtabstop = 2
-set.tabstop = 2
+set.shiftwidth = 4
+set.softtabstop = 4
+set.tabstop = 4