Fix MerkleProof natspec comments (#1083)

contracts/MerkleProof.sol

@@ -1,15 +1,15 @@
 pragma solidity ^0.4.24;
 
 
-/*
+/**
  * @title MerkleProof
  * @dev Merkle proof verification based on
  * https://github.com/ameensol/merkle-tree-solidity/blob/master/src/MerkleProof.sol
  */
 library MerkleProof {
-  /*
+  /**
    * @dev Verifies a Merkle proof proving the existence of a leaf in a Merkle tree. Assumes that each pair of leaves
-   * and each pair of pre-images is sorted.
+   * and each pair of pre-images are sorted.
    * @param _proof Merkle proof containing sibling hashes on the branch from the leaf to the root of the Merkle tree
    * @param _root Merkle root
    * @param _leaf Leaf of Merkle tree