Fixed a typo. Worked on mac, failed on linux

Author: craftybones

test/testMachine.js

@@ -1,7 +1,7 @@
 const assert = require('assert');
 const Machine = require('../src/machine.js');
 const InvalidInstructionException = require('../src/commands/invalidInstructionException.js');
-const StackUnderFlowException = require('../src/stackUnderFlowException.js');
+const StackUnderFlowException = require('../src/stackUnderflowException.js');
 
 const stitch = lines => lines.join('\n');