[MLIR] Fix tensor shapes in Toy chapter 1

In Toy tutorial chapter 1, multiply_transpose() is called with b<2, 3>
and c<3, 2> when both parameters should have the same shape. This commit
fixes this by instead using c and d as parameters and fix a comment typo
where c and d are mentioned to have shape <2, 2> when they actually have
shape <3, 2>.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D142622