Fix incorrect connection string format in README.md example

[yukiharada1228]

The connection string example in README.md is incorrect.

Currently, README.md shows:

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:

CONNECTION_STRING = "postgresql+psycopg3://langchain:langchain@localhost:6024/langchain"

to:

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

[averikitsch]

Thank you!