Fix Colab link in parent document retrieval notebook

[rvernica]

In the Parent Document Retrieval notebook, the Colab link pointed to the Hybrid Search Colab notebook. This PR fixes the link to point to the Parent Document Retrieval Colab notebook.

[davidhou17]

Hi @rvernica, we're currently not accepting public contributions to this repo but thanks for raising this! Created a separate PR to fix this (and something else) #31