Remove non-existent Mongo adapter from README.

The package only ships Redis and MinIO adapters, so the docs should not reference a mongo extra.

Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -7,11 +7,10 @@ Various python repository interfaces exposed as a python package.
 This package supports interacting with multiple different backends:
 
 - Redis
-- Mongo
 - MinIO
 
 To add support for a specific backend install this package with one or more of these optional packages:
 
 ```bash
-uv add python-repositories[redis, mongo, minio]
+uv add python-repositories[redis, minio]
 ```