Typo fix at MonadTransformers.lean

[JiechengZhao]

Maybe typo. The functions defined in the previous section shall be right, but here, the functions will give unwanted result.

Before.

 bin/
│    hello.hash
│    hello.trace
│    hello

After, should be.

├── bin/
│   ├── hello.hash
│   ├── hello.trace
│   ├── hello