fix topic error

[honeywest]

balance not index

https://substrate.dev/substrate-contracts-workshop/#/2/creating-an-event

https://github.com/OpenZeppelin/openzeppelin-contracts/blob/538b6d21b15733601f9193af5b9f662b94f16ea1/contracts/token/ERC20/IERC20.sol#L70-L76

[codecov-io]

Codecov Report

Merging #768 (bd39124) into master (f32c3af) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #768      +/-   ##
==========================================
- Coverage   67.50%   67.48%   -0.02%     
==========================================
  Files         163      163              
  Lines        7502     7502              
==========================================
- Hits         5064     5063       -1     
- Misses       2438     2439       +1     

Impacted Files	Coverage Δ
crates/storage/src/traits/impls/fuzz_tests.rs	96.87% <0.00%> (-3.13%)	⬇️

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[Robbepop]

thanks! it is good to have better parity with OpenZepelin