Consistent spellings of reentrant

Changed `rentrancy_lock` variable to `reentrancy_lock`

contracts/ReentrancyGuard.sol

@@ -11,7 +11,7 @@ contract ReentrancyGuard {
   /**
    * @dev We use a single lock for the whole contract.
    */
-  bool private rentrancy_lock = false;
+  bool private reentrancy_lock = false;
 
   /**
    * @dev Prevents a contract from calling itself, directly or indirectly.
@@ -22,10 +22,10 @@ contract ReentrancyGuard {
    * wrapper marked as `nonReentrant`.
    */
   modifier nonReentrant() {
-    require(!rentrancy_lock);
-    rentrancy_lock = true;
+    require(!reentrancy_lock);
+    reentrancy_lock = true;
     _;
-    rentrancy_lock = false;
+    reentrancy_lock = false;
   }
 
 }