dev-contributing.txt: Update google-java-format version to 1.6
dev-contributing.txt: Update google-java-format version to 1.6

Version 1.6 is used since change Iedba3f24a, but the documentation
update was overlooked.

Change-Id: I8e2215b0c06efec7f6f71268de7cbbd8d9723f9e

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