docs: fix broken link to assets-in-binary example

The referenced `assets-in-binary/example02` directory no longer exists
in the gin-gonic/examples repository (returns 404). The example now lives
directly under `assets-in-binary`. Update the link accordingly and format
it as a Markdown link to match the surrounding example references.

docs/doc.md

@@ -2085,7 +2085,7 @@ func main() {
 }
 ```
 
-See a complete example in the `https://github.com/gin-gonic/examples/tree/master/assets-in-binary/example02` directory.
+See a complete example in the [`assets-in-binary`](https://github.com/gin-gonic/examples/tree/master/assets-in-binary) directory.
 
 ## Server Configuration