[BUG] mnist-tool README links to missing MNIST documentation

[x-n2o]

Describe the bug

Tools/mnist-tool/README.md links to Libraries/MNIST/README.md, but that path is missing from the main branch — the library was renamed to Libraries/MLXMNIST in #151 and the link was never updated — so it resolves to a GitHub 404.

To Reproduce

1. Open Tools/mnist-tool/README.md on main.

2. Find the entry:

   See the [MNIST README.md](../../Libraries/MNIST/README.md).

3. Click MNIST README.md.

4. Observe that Libraries/MNIST/README.md is not found.

Expected behavior

The link should resolve to the existing Libraries/MLXMNIST/README.md.

Desktop (please complete the following information):

• OS Version: N/A
• Device: N/A
• Version: main

Additional context

Surfaced by the markdown link check proposed in #479. Unlike the targets that moved to the mlx-swift-lm package, this one still exists in the repository under the renamed path, so it is a one-line link fix.