added missing cmd arg for bart cnn ft

Summary: Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/930

Differential Revision: D18763707

fbshipit-source-id: 453a877f5bb39c5afcf7f9bc101019b1be4a0a60

examples/bart/README.cnn.md

@@ -51,6 +51,7 @@ CUDA_VISIBLE_DEVICES=0,1,2,3,4,5,6,7 python train.py cnn_dm-bin \
     --max-tokens $MAX_TOKENS \
     --task translation \
     --source-lang source --target-lang target \
+    --truncate-source \
     --layernorm-embedding \
     --share-all-embeddings \
     --share-decoder-input-output-embed \
@@ -64,7 +65,7 @@ CUDA_VISIBLE_DEVICES=0,1,2,3,4,5,6,7 python train.py cnn_dm-bin \
     --clip-norm 0.1 \
     --lr-scheduler polynomial_decay --lr $LR --total-num-update $TOTAL_NUM_UPDATES --warmup-updates $WARMUP_UPDATES \
     --fp16 --update-freq $UPDATE_FREQ \
-    --skip-invalid-size-inputs-valid-test\
+    --skip-invalid-size-inputs-valid-test \
     --find-unused-parameters;
 ```
 Above is expected to run on `1` node with `8 32gb-V100`.