Add explicit “expert” term in request comment

[remi]

We had a small inconsistency between how we represent “expert” and “learner” reviewers.

Before

%SelectedUser{metadata: %{stack: "graphql"}, type: "stack", username: "foo"}
%SelectedUser{metadata: %{stack: "graphql"}, type: "learner", username: "foo"}

After

%SelectedUser{metadata: %{stack: "graphql"}, type: "expert", username: "foo"}
%SelectedUser{metadata: %{stack: "graphql"}, type: "learner", username: "foo"}

[mirego-builds]

🦀 Requesting reviewers for this pull request:

• @garno (contributor with 5 commits in the last 90 days and 5 commits overall)
• @simonprev (reviewer for the elixir stack)