### 5.43.20 Exchange two rows : rowSwap

rowSwap takes three arguments : a matrix A and two integers
n1 and n2.

rowSwap returns the matrix obtained by exchanging in A, the row of
index n1 with the row of index n2.

Input :

rowSwap([[1,2],[3,4]],0,1)

Output :

[[3,4],[1,2]]