Update Slack invitation link (#1319)
Update Slack invitation link (#1319)

Fixes https://github.com/bazelbuild/bazel-website/issues/251
bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration,bazelbuild/continuous-integration