docs(l2-rollup): rm the duplicated L1_BEACON_URL

[jsvisa]

Description

rm the duplicated L1_BEACON_URL, it's already defined here

docs/create-l2-rollup-example/.example.env

Lines 8 to 14 in f8ea615

	# Option 1: Public endpoint (no API key required - recommended for testing)
	L1_RPC_URL="https://ethereum-sepolia-rpc.publicnode.com"
	L1_BEACON_URL="https://ethereum-sepolia-beacon-api.publicnode.com"
	# Option 2: Private endpoint (requires API key from Infura/Alchemy)
	# L1_RPC_URL="https://eth-sepolia.g.alchemy.com/v2/YOUR_API_KEY"
	# L1_BEACON_URL="https://ethereum-sepolia-beacon-api.publicnode.com"

Tests

Additional context

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[jsvisa]

@krofax ptal