Fix error message for Object.observe accept argument

src/messages.js

Index: src/messages.js
diff --git a/src/messages.js b/src/messages.js
index 07d67192183b36e4ab7658c0b64bd32139fe8c87..62429cfd2fd57252f1ebaab04043dcf32b5550c1 100644
--- a/src/messages.js
+++ b/src/messages.js
@@ -81,7 +81,7 @@ var kMessages = {
   observe_non_object:            ["Object.", "%0", " cannot ", "%0", " non-object"],
   observe_non_function:          ["Object.", "%0", " cannot deliver to non-function"],
   observe_callback_frozen:       ["Object.observe cannot deliver to a frozen function object"],
-  observe_invalid_accept:        ["Object.observe accept must be an array of strings."],
+  observe_invalid_accept:        ["Third argument to Object.observe must be an array of strings."],
   observe_type_non_string:       ["Invalid changeRecord with non-string 'type' property"],
   observe_perform_non_string:    ["Invalid non-string changeType"],
   observe_perform_non_function:  ["Cannot perform non-function"],