Fix `;duel accept` error msg.

Person 1: `;duel challenge @Person2`
TLE: `@Person1 is challenging @Person2 to a 1700 rated duel!`
Person 1: `;duel accept`
TLE: `@Person1, you are not being challenged.`
Person 1: `;duel withdraw`
TLE: `@Person1 withdrew a challenge to Person2.`

Imo `@Person2 didn't accepted your challenge yet.` is correct. One can also use it to remind someone of the duel.

Locate the `;duel accept` command handler and the current “not being challenged” response. Compare it with the issue’s requested wording, update the error message, and verify that accepting without an active challenge produces the intended response.

Written by the indexing model from the issue text.