Fix broken link to the Fuel Toolchain section in the book
- [Introduction](./introduction/index.md)
- [Installation](./introduction/installation.md)
- [Sway Quickstart](./introduction/sway_quickstart.md)
- - [The Fuel Toolchain](./introduction/fuel-toolchain.md)
+ - [The Fuel Toolchain](./introduction/fuel_toolchain.md)
- [A Forc Project](./introduction/forc_project.md)
- [Standard Library](./introduction/standard_library.md)
- [Examples](./examples/index.md)
