docs: adjust typo on instructions for eslint-config-turbo legacy config (#10717)
### Description
Seems like this last example was copied from
https://github.com/vercel/turborepo/blob/main/packages/eslint-plugin-turbo/README.md,
but forgot this small adjustment for the legacy config.
### Testing Instructions
Configuring a legacy `eslintrc*` file, inheriting the default
`eslint-config-turbo` rules, and customizing rules on top of that should
work by copying the example.
Co-authored-by: Anthony Shew <anthony.shew@vercel.com>