…#260)

…to default port used in tests

---------

Co-authored-by: Averi Kitsch <[email protected]>

The connection string example in README.md is incorrect.

Currently, README.md shows:
```python
CONNECTION_STRING = "postgresql+psycopg3://langchain:langchain@localhost:6024/langchain"
```

However, this format causes the following error:
```
NoSuchModuleError: Can't load plugin: sqlalchemy.dialects:postgresql.psycopg3
```

The actual example files (`examples/vectorstore.ipynb`,
`examples/pg_vectorstore.ipynb`, etc.) use the correct format
`postgresql+psycopg://`, which works correctly.

The `examples/vectorstore.ipynb` also explicitly states: "the driver
name is `psycopg` not `psycopg3`, but it'll use `psycopg3`."

**Proposed fix:**
Line 46 in README.md should be changed from:
```python
CONNECTION_STRING = "postgresql+psycopg3://langchain:langchain@localhost:6024/langchain"
```

to:
```python
CONNECTION_STRING = "postgresql+psycopg://langchain:langchain@localhost:6024/langchain"
```

### Summary

When a database schema is specified, areindex and adrop_vector_index in
AsyncPGVectorStore do not take effect.

Fixes: #269

Co-authored-by: jay.li <[email protected]>