Correct some spellings.

Test: mma
Change-Id: Icc49a4ee76f12d302ed18982d334f2f70b7263a8
diff --git a/omaha_request_action.cc b/omaha_request_action.cc
index 6354641..78d8d50 100644
--- a/omaha_request_action.cc
+++ b/omaha_request_action.cc
@@ -180,7 +180,7 @@
                        const string arg_name,
                        const string prefs_key) {
   // There's nothing wrong with not having a given cohort setting, so we check
-  // existance first to avoid the warning log message.
+  // existence first to avoid the warning log message.
   if (!prefs->Exists(prefs_key))
     return "";
   string cohort_value;