Fix kokoro config syntax error

Change-Id: Ic5de1c2233ec866676af6dc8bca23a89ff038807

Author: Jon Wayne Parrott

.kokoro/system_tests_appengine.cfg

@@ -11,5 +11,5 @@ env_vars: {
 
 env_vars: {
     key: "NOX_SESSION"
-    value: gae"
+    value: "gae"
 }