ART: Added comments, fixed typos in Checker

Change-Id: I1ff12940035845c1a586d4df826efc794088bdc9
diff --git a/tools/checker_test.py b/tools/checker_test.py
index 9b04ab0..9bdd0ad 100755
--- a/tools/checker_test.py
+++ b/tools/checker_test.py
@@ -23,7 +23,7 @@
 
 # The parent type of exception expected to be thrown by Checker during tests.
 # It must be specific enough to not cover exceptions thrown due to actual flaws
-# in Checker..
+# in Checker.
 CheckerException = SystemExit