。
The choice of example sentence was poor.
Literal
Example-sentence [possessive-の] selection [subject-が] was-bad [copula-です].
選定 'selection, choosing' is more formal than 選び. まずかった 'was bad, was a poor choice' — the past tense of まずい in its sense of 'bad, inadequate, ill-advised' rather than 'bad-tasting.' A self-critical admission about a language example.