docs: update API Reference Link in /docs/how_to/vectorstore_retriever/ (#27477)
Description: updated docs
[here](https://python.langchain.com/docs/how_to/vectorstore_retriever/#:~:text=VectorStoreRetriever)
for creating VectorStoreRetrievers. The URL was missing a `.base`, and
now works as expected.
This was a fix for Issue #27196