docs: Fix example extension directory structure (#16424)

Add language-specific subdirectory in example directory structure, since
that's the requisite structure - see `extensions/languages.md`

Release Notes:

- N/A

docs/src/extensions/developing-extensions.md

@@ -32,6 +32,7 @@ my-extension/
   src/
     lib.rs
   languages/
+    my-language/
       config.toml
       highlights.scm
   themes/