DOCS: fix link to question answering (#13806)

first link in
[overview](https://python.langchain.com/docs/use_cases/question_answering/code_understanding#overview)

docs/docs/use_cases/question_answering/code_understanding.ipynb

@@ -28,7 +28,7 @@
     "\n",
     "## Overview\n",
     "\n",
-    "The pipeline for QA over code follows [the steps we do for document question answering](/docs/docs/use_cases/question_answering), with some differences:\n",
+    "The pipeline for QA over code follows [the steps we do for document question answering](/docs/use_cases/question_answering), with some differences:\n",
     "\n",
     "In particular, we can employ a [splitting strategy](https://python.langchain.com/docs/integrations/document_loaders/source_code) that does a few things:\n",
     "\n",