Neural network Introduction step 5 not running(Pull req #10)

[Ritvik-Sapra]

In the step 5 of neural networks introduction, "" is asked to change with 'accuracy', however later on we have been using 'acc' as a variable for accuracy. Due to this typo, the code generates an error which might be difficult for a beginner to resolve. After replacing "" with 'acc' instead of 'accuracy', the code worked fine.