Update fgsm_tutorial.py (#2082)

Added a comma

Author: Svetlana Karslioglu

beginner_source/fgsm_tutorial.py

@@ -17,7 +17,7 @@
 machine learning. You may be surprised to find that adding imperceptible
 perturbations to an image *can* cause drastically different model
 performance. Given that this is a tutorial, we will explore the topic
-via example on an image classifier. Specifically we will use one of the
+via example on an image classifier. Specifically, we will use one of the
 first and most popular attack methods, the Fast Gradient Sign Attack
 (FGSM), to fool an MNIST classifier.