[mlir] [docs] Broken link in MLIR Toy docs

In the Ch6 of the Toy Example for MLIR, there is a broken link.  If ones goes to [the page for Chapter 6](https://mlir.llvm.org/docs/Tutorials/Toy/Ch-6/), and click on the link "Conversion to the LLVM IR Dialect", one will see it takes you to a page that no longer exists.

I believe this should actually be [this link](https://mlir.llvm.org/docs/TargetLLVMIR/).

Note to reviewers that this is my first submitted patch to LLVM, and using the phabricator system, so there is a higher risk that I have made an error, and brief feedback on these patch notes would be appreciated.

Reviewer rational: Users who git blame say contributed to the tutorial.

I believe that automated tests on these markdown docs could reduce the risk of this kind of error occurring again.  For example, [this Python package](https://pypi.org/project/linkcheckmd/) checks for broken markdown links.  However, I am unsure where in the existing testing infrastructure this could go, I am only somewhat familiar with the C++ side.

Reviewed By: Mogball

Differential Revision: https://reviews.llvm.org/D133977