typo in an equation in introduction.ipynb

[malmassari]

There is a typo in Line 52 in the equation in the introduction.ipynb file in the discussion of the recursive implementation of multiplying two integer numbers. The original equation is (Line 52):
x.y = (10^ {n/2}.a + b) \times (10^ {n/2}.c + d) = 10^n.ab + 10^{n/2}.(a.d + b.c) + b.d

The correct equation should be:
x.y = (10^ {n/2}.a + b) \times (10^ {n/2}.c + d) = 10^n.ac + 10^{n/2}.(a.d + b.c) + b.d