Add linux_chromium_msan_rel_ng trybot to clang update instruction.

BUG=559530
NOTRY=true

Review URL: https://codereview.chromium.org/1470023002

Cr-Commit-Position: refs/heads/master@{#361189}

docs/updating_clang.md

@@ -17,8 +17,9 @@
     git cl try -m tryserver.chromium.linux -b linux_chromium_gn_dbg -b
     linux_chromium_chromeos_dbg_ng -b linux_chromium_asan_rel_ng -b
     linux_chromium_chromeos_asan_rel_ng -b android_clang_dbg_recipe -b
-    linux_chromium_trusty32_rel -b linux_chromium_rel_ng && git cl try -m
-    tryserver.blink -b linux_blink_rel`
+    linux_chromium_trusty32_rel -b linux_chromium_rel_ng -b
+    linux_chromium_msan_rel_ng && git cl try -m tryserver.blink -b
+    linux_blink_rel`
 1.  Commit roll CL from the first step
 1.  The bots will now pull the prebuilt binary, and goma will have a matching
     binary, too.