Add a backup web3 provider

It's a good idea to add a backup web3 provider in case your main one goes down. You can do this by simply repeating the --web3-url parameter on launch.

For example, if your primary eth1 node is a local Geth, but you want to use Infura as a backup you would run:

./run-mainnet-beacon-node.sh \
 --web3-url="ws://127.0.0.1:8546" \
 --web3-url="wss://mainnet.infura.io/ws/v3/..."