Fix Solidity interface for SubstrateEcdsa

[akru]

Pull Request Summary

Just a cosmetic adjustments for Solidity interface of SubstrateEcdsa.

Check list

• [ ] contains breaking changes
• [ ] adds new feature
• [ ] modifies existing feature (bug fix or improvements)
• [ ] relies on other tasks
• [ ] documentation changes
• [ ] tests and/or benchmarks are included
• [ ] changed API client type definition or chain metadata

This pull request makes the following changes:

Adds

• (ex: Add feature A)

Fixes

• (ex: Fix validation function)

Changes

• (ex: Change document B)

To-dos

*Feel free to remove this section if it's not applicable

• [ ] (ex: add user list)

[akru]

It was trivial change

[Dinonard]

@akru just heads-up, you merged it into polkadot-v0.9.17