bisect-kit: cros_util: fix variable name

BUG=b:185874058
BUG=b:185978505
TEST=manually

Change-Id: I07820a5d9daffbae38f99539d32b651798426d43
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/bisect-kit/+/2896426
Tested-by: Zheng-Jie Chang <zjchang@chromium.org>
Auto-Submit: Zheng-Jie Chang <zjchang@chromium.org>
Reviewed-by: Kuang-che Wu <kcwu@chromium.org>
Commit-Queue: Kuang-che Wu <kcwu@chromium.org>
diff --git a/bisect_kit/cros_util.py b/bisect_kit/cros_util.py
index c155570..fc64112 100644
--- a/bisect_kit/cros_util.py
+++ b/bisect_kit/cros_util.py
@@ -2142,7 +2142,7 @@
     # file, the branch with "smallest" number guarantees where the release is.
 
     def branch_key(s):
-      if s == master_refname:
+      if s == main_refname:
         return 0, 0, 0
       m = re.search(r'-(\d+)\.B$', s)
       if m: