nvim: Fix build of markdown-preview

For some versions apparently markdown preview could not be built anymore
with the suggested installation in the readme. Instead it requires the
build function to be called through a string from lazy.

See:
https://github.com/iamcco/markdown-preview.nvim/issues/690

nvim/.config/nvim/lua/plugins/prose.lua

@@ -127,9 +127,7 @@ local prose_plugs = {
 	{
 		"iamcco/markdown-preview.nvim",
 		cond = vim.fn.executable("deno") == 0,
-		build = function()
-			vim.fn["mkdp#util#install"]()
-		end,
+		build = ":call mkdp#util#install()",
 		version = false,
 		ft = md_like,
 		cmd = { "MarkdownPreviewToggle", "MarkdownPreview", "MarkdownPreviewStop" },