docs: update pg8000 link to new repository location (#513)

The former link doesn't exist anymore. This is the new link to pg8000
that is referenced from the PyPI page: https://pypi.org/project/pg8000/.

Similar change in the CloudSQL connector:
GoogleCloudPlatform/cloud-sql-python-connector#1368.

Author: rhatgadkar-goog

README.md

@@ -41,7 +41,7 @@ Using an AlloyDB Connector provides the following benefits:
 The AlloyDB Python Connector is a package to be used alongside a database driver.
 Currently supported drivers are:
 
-* [`pg8000`](https://github.com/tlocke/pg8000)
+* [`pg8000`](https://codeberg.org/tlocke/pg8000)
 * [`asyncpg`](https://magicstack.github.io/asyncpg)
 
 ## Installation