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/java.section.md

@@ -96,7 +96,7 @@ let
   something = (pkgs.something.override { jre = my_jre; });
   other = (pkgs.other.override { jre = my_jre; });
 in
-  "..."
+  <...>
 ```
 
 You can also specify what JDK your JRE should be based on, for example