docs(doc): Change custom icons to columns config

doc/oil.txt

@@ -17,7 +17,7 @@ OPTIONS                                                              *oil-option
       -- Id is automatically added at the beginning, and name at the end
       -- See :help oil-columns
       columns = {
-        "icon",
+        { "icon", default_file = "", directory = " " },
         -- "permissions",
         -- "size",
         -- "mtime",
@@ -37,11 +37,6 @@ OPTIONS                                                              *oil-option
         conceallevel = 3,
         concealcursor = "n",
       },
-      -- Custom icons
-      icons = {
-        default_file = "",
-        directory = " ",
-      },
       -- Oil will take over directory buffers (e.g. `vim .` or `:e src/`
       default_file_explorer = true,
       -- Restore window options to previous values when leaving an oil buffer