Add more descriptive variable name for root path.

Looking at https://github.com/expressjs/generator/commit/19eb55d4f276c2d51f7d5342725f526080608611, it appears like the previous version had the `routes` variable as a namespace for multiple routers within the same file, i.e. `routes.index`. Since this is not the case anymore, I think `index` could be a better name for the variable and could help avoid confusion.