Fixed typo CHannel -> Channel (#73)

(I realize that I made a typo in my commit message - oh the irony)

src/DiscordClient.php

@@ -310,7 +310,7 @@ private function mapBadDocs($cls)
             case '\RestCord\Model\Guild\User':
             case '\RestCord\Model\Channel\User':
                 return '\RestCord\Model\User\User';
-            case '\RestCord\Model\CHannel\ISO8601':
+            case '\RestCord\Model\Channel\ISO8601':
                 return '\DateTimeImmutable';
             default:
                 return $cls;