fix(pgvector): similarity_search should return embeddings ordered by distance

**Describe the bug**
The similarity search does not return the correct results, rather it returns DESC which don't make sense.

This PR seems to be the culprit, I reverted and am now getting the correct answers.
https://github.com/Abraxas-365/langchain-rust/pull/224

**To Reproduce**
Steps to reproduce the behavior:
1. Run one of the postgres examples you wont get the most relevant or closest response