fix(docs): correct NotFoundError cross-reference in assistant.md

Replaced broken `pinecone.exceptions.NotFoundError` Sphinx cross-references
with the correct `pinecone.errors.exceptions.NotFoundError` path.
`pinecone/exceptions.py` is a backcompat shim that does not export
`NotFoundError`; the canonical location is `pinecone/errors/exceptions.py`.

Author: jhamon

docs/how-to/assistant.md

@@ -97,12 +97,12 @@ pc.assistant.delete_file(
 )
 ```
 
-Raises {exc}`~pinecone.exceptions.NotFoundError` if the file does not exist.
+Raises {exc}`~pinecone.errors.exceptions.NotFoundError` if the file does not exist.
 
 ## Delete an assistant
 
 ```python
 pc.assistant.delete(name="my-assistant")
 ```
 
-Raises {exc}`~pinecone.exceptions.NotFoundError` if the assistant does not exist.
+Raises {exc}`~pinecone.errors.exceptions.NotFoundError` if the assistant does not exist.