Staker.sol: TODO add link in comment

[code423n4]

Handle

hickuphh3

Vulnerability details

Impact

In _calculateFloatPerSecond(), there is an outstanding TODO in L437

@dev to see below math in latex form see TODO add link

The missing URL seems to be the one provided in the README on float token rate issuance.

Recommended Mitigation Steps

Update / finish up the TODO.

[JasoonS]

Thanks