groupmedia_update_8001 uses incorrect storage ID

Problem/Motivation

`groupmedia_update_8001` uses the `group_relationship_type` storage type, however group 2.x uses `group_content_type` for the storage type.

Steps to reproduce

Install and enable version 3.0-alpha4. Add a group type and configure groupmedia to enable tracking. Upgrade to 3.0-alpha8. Run updates (update.php or drush updb). You'll receive the error `[error] The "group_relationship_type" entity type does not exist.`.

Proposed resolution

Use the correct storage ID `group_content_type`.