[test] Fix comment in test configs

NOTRY=true
TBR=sergiyb@chromium.org

Bug: chromium:830557
Change-Id: I598a8591559c489944da6aa06fea793aaed0fa6e
Reviewed-on: https://chromium-review.googlesource.com/c/1296479Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56900}

infra/testing/builders.pyl

@@ -1510,7 +1510,7 @@
     ],
   },
   ##############################################################################
-  # Clusterfuzz.
+  # Branches.
   'V8 Linux - beta branch': {
     'swarming_task_attrs': {
       'expiration': 14400,