Update cq.cfg to use new name for committers group.

The old name is going away.

R=tandrii@chromium.org
NOTRY=True
BUG=

Review-Url: https://codereview.chromium.org/2671593004
Cr-Commit-Position: refs/heads/master@{#447993}

infra/config/cq.cfg

@@ -13,7 +13,7 @@ rietveld {
 
 verifiers {
   reviewer_lgtm {
-    committer_list: "external/project-chromium-committers"
+    committer_list: "project-chromium-committers"
     dry_run_access_list: "project-chromium-tryjob-access"
   }