treewide: Switch markdown placeholder from "..." to <...>

We use angle brackets since they look a lot like a placeholder while
also being valid nix code, as suggested by roberth here: https://github.com/NixOS/nixpkgs/pull/299554#discussion_r1541797970

doc/languages-frameworks/agda.section.md

@@ -194,7 +194,7 @@ mkDerivation {
   version = "1.5.0";
   pname = "iowa-stdlib";
 
-  src = "...";
+  src = <...>;
 
   libraryFile = "";
   libraryName = "IAL-1.3";