updated blocksbehind logic for bsc and .env file for base

What does this PR do?

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In the syncchecker-bsc.sh file, I added a check in order to be able handle negative values in the case that the current block value is bigger than the highest block value. That way, in the CW dashboard, for the BSC Client Blocks Behind graph, the graph doesn't keep going into the negatives. In addition to that, in the README.md file for Base, I fixed the name of the .env file. It was cp ./sample-configs/.env-sample-rpc .env but should be cp ./sample-configs/.env-sample-full .env.

Motivation

License

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[ ] Yes, I have tested the PR using my local account setup (Provide any test evidence report under Additional Notes)
[ ] Mandatory for new node blueprints. Yes, I have added usage example to the README.md file in my blueprint folder
[ ] Mandatory for new node blueprints. Yes, I have implemented automated tests for all stacks in my blueprint and they pass
[ ] Mandatory for new node blueprints. Yes, I have added a reference to my README.md file to website/docs section for this feature
[ ] Yes, I have set up and ran all pre-merge quality control tools on my local machine and they don't show warnings.

For Moderators

[ ] The tests for all current blueprints successfully complete before merge?
[ ] Mandatory for new node blueprints. All pre-merge quality control tools and cdk-nag tools don't show warnings?
[ ] Mandatory for new node blueprints. The deployment test works on blank AWS account according to instructions in the README.md before merge?
[ ] Mandatory for new node blueprints. The website builds without errors?

aws-samples / aws-blockchain-node-runners