Small documentation fixes for bazel emscripten tutorial.
Small documentation fixes for bazel emscripten tutorial.

- Fix error message in step 4, it differs from what is display when following the tutorial.
- Remove cpu from cc_toolchain, it's not needed in newer versions of Bazel.

Closes #9301.

PiperOrigin-RevId: 272407254

perezd/bazel,meteorcloudy/bazel,cushon/bazel,safarmer/bazel,dslomov/bazel,davidzchen/bazel,cushon/bazel,werkt/bazel,katre/bazel,davidzchen/bazel,meteorcloudy/bazel,ulfjack/bazel,perezd/bazel,twitter-forks/bazel,dslomov/bazel,safarmer/bazel,meteorcloudy/bazel,safarmer/bazel,ButterflyNetwork/bazel,dslomov/bazel,cushon/bazel,akira-baruah/bazel,davidzchen/bazel,dslomov/bazel-windows,aehlig/bazel,katre/bazel,davidzchen/bazel,cushon/bazel,ButterflyNetwork/bazel,katre/bazel,werkt/bazel,ButterflyNetwork/bazel,werkt/bazel,ulfjack/bazel,dslomov/bazel-windows,bazelbuild/bazel,twitter-forks/bazel,bazelbuild/bazel,twitter-forks/bazel,safarmer/bazel,werkt/bazel,bazelbuild/bazel,dslomov/bazel,dslomov/bazel,perezd/bazel,safarmer/bazel,perezd/bazel,aehlig/bazel,perezd/bazel,dslomov/bazel-windows,twitter-forks/bazel,dslomov/bazel-windows,davidzchen/bazel,ButterflyNetwork/bazel,katre/bazel,ulfjack/bazel,werkt/bazel,meteorcloudy/bazel,akira-baruah/bazel,safarmer/bazel,aehlig/bazel,meteorcloudy/bazel,katre/bazel,ButterflyNetwork/bazel,cushon/bazel,ulfjack/bazel,perezd/bazel,dslomov/bazel,bazelbuild/bazel,akira-baruah/bazel,bazelbuild/bazel,katre/bazel,twitter-forks/bazel,ButterflyNetwork/bazel,meteorcloudy/bazel,werkt/bazel,dslomov/bazel,twitter-forks/bazel,akira-baruah/bazel,akira-baruah/bazel,twitter-forks/bazel,aehlig/bazel,aehlig/bazel,cushon/bazel,ulfjack/bazel,ulfjack/bazel,meteorcloudy/bazel,akira-baruah/bazel,davidzchen/bazel,perezd/bazel,davidzchen/bazel,ulfjack/bazel,aehlig/bazel,dslomov/bazel-windows,bazelbuild/bazel,dslomov/bazel-windows,aehlig/bazel