[mlir] Toy tutorial: insert terminators at the end of the loop during rewrite

When creating temporary `scf.for` loops in `toy.print` lowering, the block
insertion point was erronously set up to the beginning of the block rather than
to its end, contradicting the comment just above the insertion point change.
The code was nevertheless operational because `scf.for` was setting up its
`scf.yield` terminator in an opaque to the pattern rewriting infrastructure
way. Now that it is about to change, the problem would have been exposed and
lead to conversion failures.

Differential Revision: https://reviews.llvm.org/D80133