fix typo: that -> than

test/ReentrancyGuard.test.js

@@ -17,7 +17,7 @@ contract('ReentrancyGuard', function (accounts) {
     await expectThrow(reentrancyMock.countAndCall(attacker.address));
   });
 
-  // The following are more side-effects that intended behaviour:
+  // The following are more side-effects than intended behaviour:
   // I put them here as documentation, and to monitor any changes
   // in the side-effects.