✨ [amp-ad broadstreetads] Replaced default value for initUrl

[stevegardiner26]

This PR updates the URL to our initialization script to a more recent/widely used version for the default value.

[CLAassistant]

All committers have signed the CLA.

[calebcordry]

Thanks for maintaining this!

[stevegardiner26]

Hey @calebcordry, just a note I fixed a lint error and then I saw the note about auto-merge. Anything I need to do?

[calebcordry]

Merged!

[ampprojectbot]

Warning: disparity between this PR Percy build and its main build

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