Fix `;duel accept` error msg.

[aryanc403]

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.