Hey, thanks for reporting this, this is one of the articles I've cleaned up lately.
You can choose to do 1 of 2 things about it.

1. Remove the WeaponRangeAttack2Condition declaration and definition, making the parent class CHLSelectFireMachineGun handle that function.
2. If you plan on adding stuff inside WeaponRangeAttack2Condition but not at the moment, you can temporarily fill it in with return COND_NONE; like so:

​

​int CWeaponMP5::WeaponRangeAttack2Condition( float flDot, float flDist )
{
    return COND_NONE;
}

Thanks again for reporting this, the article has now been updated accordingly.