Replace did:key with did:foo in resolution example

[PhilippGackstatter]

Description of change

Replace did:key with did:foo in the resolution example, effectively removing the did-key dependency. This is unfortunately necessary because ferrilab/funty#7 is not resolved.

Links to any relevant issues

n/a

Type of change

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• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

Ran the example locally, successfully.

Change checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[olivereanderson]

LGTM