docs: fixed link to docs of unstructured (#32306)
In the section [How to load documents from a
directory](https://python.langchain.com/docs/how_to/document_loader_directory/)
there is a link to the docs of *unstructured*. When you click this link,
it tells you that it has moved. Accordingly this PR fixes this link in
LangChain docs directly
from: `https://unstructured-io.github.io/unstructured/#`
to: `https://docs.unstructured.io/`
