Update release_prep.sh

Fix TAG name

Updating release process to use stable tarball urls (bazel-contrib#1059)

Release version 0.9.0 (bazel-contrib#944)

Release `v0.8.0` (bazel-contrib#892)

More quoting fixes for handling paths with spaces (bazel-contrib#850)

Release `v0.7.0` (bazel-contrib#827)

Release 0.6.0 (bazel-contrib#784)

Release 0.5.1 (bazel-contrib#747)

Release 0.5.0 (bazel-contrib#740)

Docs are now auto-generated using mdbook (bazel-contrib#692)