fix XRD+XML serialisation of `Alias`

What does this PR do?

Fix a small mistake from Add aliases to webfinger request. (!778 - merged) (cc @piuvas)

Tested:

$ curl -H 'Accept: application/xrd+xml' http://localhost:3000/.well-known/webfinger?resource=acct:one@localhost:3000|xmllint -format -

<XRD xmlns="http://docs.oasis-open.org/ns/xri/xrd-1.0">
  <Subject>acct:one@localhost:3000</Subject>
  <Link rel="self" type="application/activity+json" href="http://localhost:3000/users/9uf94m486g6j0001"/>
  <Link rel="http://webfinger.net/rel/profile-page" type="text/html" href="http://localhost:3000/@one"/>
  <Link rel="http://ostatus.org/schema/1.0/subscribe" template="http://localhost:3000/authorize-follow?acct={uri}"/>
  <Alias>http://localhost:3000/@one</Alias>
</XRD>

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requested review from @fEmber, @Marie, and @tess

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mentioned in commit 16909ed6

merged