Fix Typo in README 
Fix Typo in README 

Changed --start_index to --start-index for fetch_papers.py, in README
karpathy/arxiv-sanity-preserver,shencebebetterme/arxiv-sanity-preserver,karpathy/arxiv-sanity-preserver,karpathy/arxiv-sanity-preserver,shencebebetterme/arxiv-sanity-preserver,shencebebetterme/arxiv-sanity-preserver,shencebebetterme/arxiv-sanity-preserver