docs: Update Retrievers and Runnable links in Retrievers subsection of retrievers.ipynb (#27815)

**Description:** Update outdated links for `Retrievers` and `Runnable`
in Retrievers subsection of `retrievers.ipynb`

Author: sifatj

docs/docs/tutorials/retrievers.ipynb

@@ -309,9 +309,9 @@
     "\n",
     "## Retrievers\n",
     "\n",
-    "LangChain `VectorStore` objects do not subclass [Runnable](https://python.langchain.com/api_reference/core/index.html#module-langchain_core.runnables), and so cannot immediately be integrated into LangChain Expression Language [chains](/docs/concepts/lcel).\n",
+    "LangChain `VectorStore` objects do not subclass [Runnable](https://python.langchain.com/api_reference/core/index.html#langchain-core-runnables), and so cannot immediately be integrated into LangChain Expression Language [chains](/docs/concepts/lcel).\n",
     "\n",
-    "LangChain [Retrievers](https://python.langchain.com/api_reference/core/index.html#module-langchain_core.retrievers) are Runnables, so they implement a standard set of methods (e.g., synchronous and asynchronous `invoke` and `batch` operations) and are designed to be incorporated in LCEL chains.\n",
+    "LangChain [Retrievers](https://python.langchain.com/api_reference/core/index.html#langchain-core-retrievers) are Runnables, so they implement a standard set of methods (e.g., synchronous and asynchronous `invoke` and `batch` operations) and are designed to be incorporated in LCEL chains.\n",
     "\n",
     "We can create a simple version of this ourselves, without subclassing `Retriever`. If we choose what method we wish to use to retrieve documents, we can create a runnable easily. Below we will build one around the `similarity_search` method:"
    ]