Fix the link to the Python wrapper.

Change-Id: I9fdb4ce637857acb6d7c9979a910bf35db99fa75
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62870
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>