doc: Replace Tree-sitter `maintained` with `all` in README (#3088)

`maintained` option is not supported anymore https://github.com/nvim-treesitter/nvim-treesitter/issues/2887

README.md

@@ -92,7 +92,7 @@ lvim.builtin.notify.active = true
 lvim.builtin.terminal.active = true
 
 -- Treesitter parsers change this to a table of the languages you want i.e. {"java", "python", javascript}
-lvim.builtin.treesitter.ensure_installed = "maintained"
+lvim.builtin.treesitter.ensure_installed = "all"
 lvim.builtin.treesitter.ignore_install = { "haskell" }
 
 -- Disable virtual text