README points to 404

[ecp4224]

The Token Vesting contract link points to a missing page

[zachlysobey]

I was about to fix the link, but it wasn't as straight-forward as I'd hoped.

openzepplin-solidity@master currently has this file in contracts/drafts

But... this project depends on a version of oz-solidity which is over a year old. Moreover, its not a published version or tag -- its pointing at a git commit hash.

"zeppelin-solidity": "git+https://git@github.com/OpenZeppelin/zeppelin-solidity.git#8e01dd14f9211239213ae7bd4c6af92dd18d4ab7"

https://github.com/OpenZeppelin/token-vesting-ui/blob/master/package.json#L21
https://github.com/OpenZeppelin/openzeppelin-solidity/tree/8e01dd14f9211239213ae7bd4c6af92dd18d4ab7

I guess it might be sensible to just update the link to point to the file at that commit hash, but once that dependency gets updated, it will likely become incorrect again.