[Guido's Gorgeous Lasagna] Update introduction.md (#3732)

Corrected example/sum on line 107 to 11.

Author: BethanyG

exercises/concept/guidos-gorgeous-lasagna/.docs/introduction.md

@@ -104,7 +104,7 @@ def add_two_numbers(number_one, number_two):
 # the variable will also return the value.
 >>> sum_with_return = add_two_numbers(5, 6)
 >>> print(sum_with_return)
-7
+11
 ```