Upgrade recommended version of buildifier to 3.0.0

The noteworthy formatting change [1] could be [2], beside the other [3].

Experience should tell soon if/how these warnings may impact buildifier
use within Gerrit.

[1] https://github.com/bazelbuild/buildtools/releases/tag/3.0.0
[2] "Warnings for macro functions without name parameters"
[3] "Improved multi-file analysers"

Change-Id: I299155e0b221845420e296371c3cf80efad1fc80

Documentation/dev-contributing.txt

diff --git a/Documentation/dev-contributing.txt b/Documentation/dev-contributing.txt
index c5ede98..b91e86e 100644
--- a/Documentation/dev-contributing.txt
+++ b/Documentation/dev-contributing.txt
@@ -171,7 +171,7 @@
 link:https://github.com/google/google-java-format[`google-java-format`]
 tool (version 1.7), and to format Bazel BUILD, WORKSPACE and .bzl files the
 link:https://github.com/bazelbuild/buildtools/tree/master/buildifier[`buildifier`]
-tool (version 2.2.1). Unused dependencies are found and removed using the
+tool (version 3.0.0). Unused dependencies are found and removed using the
 link:https://github.com/bazelbuild/buildtools/tree/master/unused_deps[`unused_deps`]
 build tool, a sibling of `buildifier`.