Add example for dependency with multiple extras (#9138)

The currently existing example on how to install dependency extras only shows a single dependency:
`fastapi = {version="^0.92.0", extras=["all"]}`

Since right before this example it is shown how to install multiple extras using the command line interface, this can be confusing and lead to hard to understand missing dependencies:
`poetry install --extras "mysql pgsql"`

Note that the CLI has quotes around both extras and no commas, while in pyproject.toml each extra *must* be quoted and comma-separated, which is currently not documented.

(cherry picked from commit 9cda0435)