Update clang update doc to include new(ish) "n" prefix on rev

Bug: 1015315
Change-Id: Ia34164d1bee32f7b217300b7afdf0dfb00682352
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2016843
Auto-Submit: Nico Weber <thakis@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734508}

docs/updating_clang.md

@@ -18,7 +18,7 @@ An archive of all packages built so far is at https://is.gd/chromeclang
     bucket to the production one. For example:
 
     ```shell
-    $ export rev=123456-abcd1234-1
+    $ export rev=n123456-abcd1234-1
     $ for x in Linux_x64 Mac Win ; do \
         gsutil.py cp -n -a public-read gs://chromium-browser-clang-staging/$x/clang-$rev.tgz \
             gs://chromium-browser-clang/$x/clang-$rev.tgz ; \