Fixes link to spec 404

[auyer]

The previous link to the singer spec yielded a 404 error, because the file SPEC.md was moved to docs/SPEC.md

There still is a SPEC.md file located in a sub folder (link to file).
If that is the intended file, send a message in the PR and I'll make the change.

Update: I've checked different taps and decided that the second option (in the striked part) was correct. I've commited the change and squashed both commits.

Description of change

Link to spec in the docs was corrected

Manual QA steps

• click the new link
• check if the docs are the ones intended

Risks

• none

Rollback steps

• revert this branch

[cmerrick]

Hi @auyer, thanks for your contribution!

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[cmerrick]

You did it @auyer!

Thank you for signing the Singer Contribution License Agreement.