Fix typos (#1082)

contracts/ECRecovery.sol

@@ -2,7 +2,7 @@ pragma solidity ^0.4.24;
 
 
 /**
- * @title Eliptic curve signature operations
+ * @title Elliptic curve signature operations
  * @dev Based on https://gist.github.com/axic/5b33912c6f61ae6fd96d6c4a47afde6d
  * TODO Remove this library once solidity supports passing a signature to ecrecover.
  * See https://github.com/ethereum/solidity/issues/864

contracts/ReentrancyGuard.sol

@@ -2,7 +2,7 @@ pragma solidity ^0.4.24;
 
 
 /**
- * @title Helps contracts guard agains reentrancy attacks.
+ * @title Helps contracts guard against reentrancy attacks.
  * @author Remco Bloemen <remco@2π.com>
  * @notice If you mark a function `nonReentrant`, you should also
  * mark it `external`.

contracts/crowdsale/Crowdsale.sol

@@ -12,7 +12,7 @@ import "../token/ERC20/SafeERC20.sol";
  * such functionality in its most fundamental form and can be extended to provide additional
  * functionality and/or custom behavior.
  * The external interface represents the basic interface for purchasing tokens, and conform
- * the base architecture for crowdsales. They are *not* intended to be modified / overriden.
+ * the base architecture for crowdsales. They are *not* intended to be modified / overridden.
  * The internal interface conforms the extensible and modifiable surface of crowdsales. Override
  * the methods to add functionality. Consider using 'super' where appropriate to concatenate
  * behavior.
@@ -197,4 +197,4 @@ contract Crowdsale {
   function _forwardFunds() internal {
     wallet.transfer(msg.value);
   }
-}
+}

contracts/crowdsale/price/IncreasingPriceCrowdsale.sol

@@ -17,7 +17,7 @@ contract IncreasingPriceCrowdsale is TimedCrowdsale {
   uint256 public finalRate;
 
   /**
-   * @dev Constructor, takes intial and final rates of tokens received per wei contributed.
+   * @dev Constructor, takes initial and final rates of tokens received per wei contributed.
    * @param _initialRate Number of tokens a buyer gets per wei at the start of the crowdsale
    * @param _finalRate Number of tokens a buyer gets per wei at the end of the crowdsale
    */

contracts/token/ERC721/ERC721Receiver.sol

@@ -18,12 +18,12 @@ contract ERC721Receiver {
    * @notice Handle the receipt of an NFT
    * @dev The ERC721 smart contract calls this function on the recipient
    * after a `safetransfer`. This function MAY throw to revert and reject the
-   * transfer. Return of other than the magic value MUST result in the 
+   * transfer. Return of other than the magic value MUST result in the
    * transaction being reverted.
    * Note: the contract address is always the message sender.
    * @param _operator The address which called `safeTransferFrom` function
    * @param _from The address which previously owned the token
-   * @param _tokenId The NFT identifier which is being transfered
+   * @param _tokenId The NFT identifier which is being transferred
    * @param _data Additional data with no specified format
    * @return `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
    */