Core/ObjectMgr: corrected error labels

Since the conditions are:
```
if (cInfo->minlevel > difficultyInfo->minlevel)
if (cInfo->minlevel > difficultyInfo->maxlevel)
```
and not:
```
if (cInfo->minlevel != difficultyInfo->minlevel)
if (cInfo->minlevel != difficultyInfo->maxlevel)
```
one might misunderstand those kind of errors, it would be better to specific a bit more.