doc: correct cargo doc command in contrib guide (#4933)

The command to build the documentation locally provided in the
Contribution Guide did not work for all crates in the project workspace.
Specifically, to build the docs for `tokio-stream` the flag `--cfg
docsrs` needs to be in the environment variable `RUSTFLAGS` in addition
to being in `RUSTDOCFLAGS`.

Additionally, there was text describing that the docs cannot be built
from the root of the workspace with a link to rust-lang/cargo#9274. That
issue has since been closed as complete and the listed commands do now
work from the root of the workspace. As such, that text has been
removed.

CONTRIBUTING.md

@@ -154,23 +154,17 @@ required for various parts of Tokio are missing. To build the documentation
 correctly, use this command:
 
 ```
-RUSTDOCFLAGS="--cfg docsrs" cargo +nightly doc --all-features
+RUSTDOCFLAGS="--cfg docsrs" RUSTFLAGS="--cfg docsrs" cargo +nightly doc --all-features
 ```
 
 To build documentation including Tokio's unstable features, it is necessary to
 pass `--cfg tokio_unstable` to both RustDoc *and* rustc. To build the
 documentation for unstable features, use this command:
 
 ```
-RUSTDOCFLAGS="--cfg docsrs --cfg tokio_unstable" RUSTFLAGS="--cfg tokio_unstable" cargo +nightly doc --all-features
+RUSTDOCFLAGS="--cfg docsrs --cfg tokio_unstable" RUSTFLAGS="--cfg docsrs --cfg tokio_unstable" cargo +nightly doc --all-features
 ```
 
-There is currently a [bug in cargo] that means documentation cannot be built
-from the root of the workspace. If you `cd` into the `tokio` subdirectory the
-command shown above will work.
-
-[bug in cargo]: https://github.com/rust-lang/cargo/issues/9274
-
 The `cargo fmt` command does not work on the Tokio codebase. You can use the
 command below instead: