Change ```eval_envs``` to ```num_eval_envs```

[Ali Rahimi-Kalahroudi]

dreamerv2/dreamerv2/train.py

Line 129 in e02ceb9

 eval_envs = [make_async_env('eval') for _ in range(eval_envs)] 

Since the number of evaluation environments is num_eval_envs, I think there should be a change at the end of this line.

[Danijar Hafner]

Thanks! Parallel training is not really tested within this code base.