Make CONTRIBUTING.md all caps as per the Charter

According to the official Charter at
https://www.openbmc.org/wp-content/uploads/sites/62/2018/03/charter_openbmc_02142018.pdf
the file must be named literally `CONTRIBUTING`.
That is, all caps.

This has been discussed in review comments for
https://gerrit.openbmc-project.xyz/9948

This commit accompanies the corresponding changes to
the docs repository, see
https://gerrit.openbmc-project.xyz/c/openbmc/docs/+/10807

Change-Id: Ia226289caa5c0d52875c011a9f31ffdf52cf30e3
Signed-off-by: Alexander Amelkin <a.amelkin@yadro.com>
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index e9cbc86..49cd88e 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -6,7 +6,7 @@
 Submitting changes via Gerrit server
 ------------------------------------
 -   Reference [OpenBMC CLA signers](https://github.com/openbmc/openbmc-tools/blob/master/emilyshaffer/cla-signers/cla-signers)
--   Reference [OpenBMC docs](https://github.com/openbmc/docs/blob/master/contributing.md#submitting-changes-via-gerrit-server)
+-   Reference [OpenBMC docs](https://github.com/openbmc/docs/blob/master/CONTRIBUTING.md#submitting-changes-via-gerrit-server)
 
 Robot Coding Guidelines
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