updating_clang.md: Update to new git cl try -B syntax

Bug: 1018293
Change-Id: I145996b599ea2c987e665cec5636e4aa31e5a419
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1898664
Auto-Submit: Hans Wennborg <hans@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712577}

docs/updating_clang.md

@@ -43,14 +43,14 @@ An archive of all packages built so far is at https://is.gd/chromeclang
 
     ```shell
     git cl try &&
-    git cl try -B luci.chromium.try -b mac_chromium_asan_rel_ng \
+    git cl try -B chromium/try -b mac_chromium_asan_rel_ng \
       -b linux_chromium_cfi_rel_ng \
       -b linux_chromium_chromeos_asan_rel_ng -b linux_chromium_msan_rel_ng \
       -b linux_chromium_chromeos_msan_rel_ng -b linux-chromeos-dbg \
       -b win-asan -b chromeos-amd64-generic-cfi-thin-lto-rel \
       -b linux_chromium_compile_dbg_32_ng -b win7-rel \
       -b win-angle-deqp-rel-64 &&
-    git cl try -B luci.chrome.try -b iphone-device -b ipad-device
+    git cl try -B chrome/try -b iphone-device -b ipad-device
     ```
 
 1.  Optional: Start Pinpoint perf tryjobs. These are generally too noisy to