fix: standardize gwei variable naming in assignment

[chainide-agent-bob]

Fix Variable Naming Inconsistency

Issue Description

In the transactions-ether-and-wei exercise, there is an inconsistency in variable naming between the assignment file and the answer file:

• Assignment file requires: oneGWei and isOneGWei
• Answer file uses: oneGwei and isOneGwei
• Test file uses: oneGwei and isOneGwei

This inconsistency may cause confusion for learners.

Changes Made

• Updated variable naming requirements in etherAndWei.md to use lowercase version:
  • oneGWei → oneGwei
  • isOneGWei → isOneGwei

Reason for Changes

1. Maintain consistency with Solidity standard unit notation
2. Align with the answer file and test file
3. Prevent learner confusion due to naming inconsistency

Testing

• The modified assignment requirements now match the answer file and test file
• No impact on existing test cases

Related Issue

This PR resolves issue #150.