fix: update fg url

[lesterli]

Summary

This PR fixes an issue from the previous PR #38 where the wrong key was used. It should be babylon_finality_gadget_rpc instead of babylonFinalityGadgetRpc in the rollup.json file.

Test Plan

first comment out the lines 13, 16, 38, run the script and check the rollup.json

make l2-op-node-restart

set BBN_FINALITY_GADGET_RPC= in .env, it should be "babylon_finality_gadget_rpc": "" in rollup.json, and output:

Stopping the OP Node...
Setting babylon_finality_gadget_rpc with empty value in rollup.json
Starting the OP Node...

set BBN_FINALITY_GADGET_RPC=11.22.33.44:50051 in .env, it should be "babylon_finality_gadget_rpc": "11.22.33.44:50051" in rollup.json, and output:

Stopping the OP Node...
Setting babylon_finality_gadget_rpc with value 11.22.33.44:50051 in rollup.json
Starting the OP Node...

Note: Revert the commented out lines