fix: deprecated SecurityToken investors function actually expects a uint256 - an index argument

[JeneaVranceanu]

Summary of Changes

SecurityToken ABI located at Web3.Utils.st20ABI expects that investors function will be called with an argument of type uint256. Our implementation of encoding investors function call crashed as we gave it no arguments.

More info: function investors(uint256 _index) external view returns (address) (link to the function in old version of the contract) was deprecated and is no longer a part of ISecurityToken.sol. Instead there are getInvestors() and getInvestorsAt(uint256 _checkpointId) (link).

Strong advice: if we will add implementations to other smart contracts cover them with links to the source as much as possible.

Test Data or Screenshots

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[yaroslavyaroslav]

I'd like to add your proposal about providing link to the specification as the one of the demands that would be necessary to pass the review process. @JeneaVranceanu