Update Slack link to use slack.bazel.build
Update Slack link to use slack.bazel.build

PiperOrigin-RevId: 253354235
Change-Id: I9006e6ab21dbe26340a6fc0ee8202474c6282bd8

bazelbuild/rules_cc,bazelbuild/rules_cc,bazelbuild/rules_cc,bazelbuild/rules_cc