Update getting_started.rst (#1188)

Summary:
Hi,

I think there is a minor mistake in the doc. `--distributed-no-spawn` argument is needed for distributed training on multiple machines without `slurm`. Otherwise, the program will start 8 jobs on each GPU, when `nproc_per_node=8`.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/1188

Differential Revision: D17627778

Pulled By: myleott

fbshipit-source-id: 35ab6b650dc1132d7cb2d150e80d2ebf0caf3e69

docs/getting_started.rst

@@ -189,4 +189,4 @@ replacing ``node_rank=0`` with ``node_rank=1`` on the second node:
         --lr 0.0005 --min-lr 1e-09 \
         --dropout 0.3 --weight-decay 0.0 --criterion label_smoothed_cross_entropy --label-smoothing 0.1 \
         --max-tokens 3584 \
-        --fp16
+        --fp16  --distributed-no-spawn