fix(gh-pages): Correct Ospec link

[bmc1995]

The previous link led to a 404 page; Ospec is no longer part of the main code base

Description

The Ospec link has been changed from https://github.com/MithrilJS/mithril.js/blob/master/ospec to https://github.com/MithrilJS/ospec

Motivation and Context

Fixes #2859

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation change

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated docs/changelog.md

[dead-claudia]

Is there a corresponding PR against next, to ensure future docs builds don't include the bad link? Or is next all okay?