Crowdsale docs improvements. (#1446)
Crowdsale docs improvements. (#1446)

* Added a small Crowdsale fallback gas warning.

* Fixed a couple typos.

OpenZeppelin/zeppelin-solidity,OpenZeppelin/openzeppelin-contracts,OpenZeppelin/openzeppelin-contracts,OpenZeppelin/zeppelin-solidity,OpenZeppelin/openzeppelin-contracts,OpenZeppelin/zep-solidity,OpenZeppelin/zep-solidity