Merge pull request #72 from jarlethorsen/patch-2

Update ex6_2.md to fix variable mismatch

Exercises/ex6_2.md

@@ -160,7 +160,7 @@ class Stock(Structure):
     def cost(self):
         return self.shares * self.price
 
-    def sell(self, nshares):
+    def sell(self, shares):
         self.shares -= shares
 ```