common_util: Fix IsRunningOnMoblab regexp type

Silly type on the regexp

TEST=Tested on moblab
BUG=chromium:814949

Change-Id: I756249f185c434a9e3779ad6f0e35bd95b6c909c
Reviewed-on: https://chromium-review.googlesource.com/1080276
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Keith Haddow <haddowk@chromium.org>
Reviewed-by: Xixuan Wu <xixuan@chromium.org>
diff --git a/common_util.py b/common_util.py
index 150881b..c2a9287 100644
--- a/common_util.py
+++ b/common_util.py
@@ -430,7 +430,7 @@
            cmd_error.rstrip())
       return False
 
-    if re.search("r[_-]+moblab", cmd_output):
+    if re.search(r"[_-]+moblab", cmd_output):
       return True
     else:
       return False