Fixed the code in Matrix Multiplication in the file docs/datastructure.rst

[RaviTejaKomma]

The matrix multiplication example in here gives wrong output:
So changed the code from

c.append([a[i][j] * b[j][i] for j in range(0, n)])

to

c.append([ sum([a[i][k] * b[k][j] for k in range(0, n)]) for j in range(0,n) ])

and also the output of the code snippet

[kushaldas]

Please squash the commit.

[RaviTejaKomma]

Hello Kushaldas. I have squashed the commits to a single commit. Thanks :)