Update custom_tools.mdx: fix link (#23292)

Wrong parantheses

docs/source/en/custom_tools.mdx

@@ -503,7 +503,7 @@ print("\n".join([f"- {a}" for a in agent.toolbox.keys()]))
 
 Note how `image_upscaler` is now part of the agents' toolbox.
 
-Let's now try out the new tools! We will re-use the image we generated in (Transformers Agents Quickstart)[./transformers_agents#single-execution-run].
+Let's now try out the new tools! We will re-use the image we generated in [Transformers Agents Quickstart](./transformers_agents#single-execution-run).
 
 ```py
 from diffusers.utils import load_image