Update a comment. (firebase#521)

functions/app/src/main/java/com/google/samples/quickstart/functions/MainActivity.java

@@ -120,7 +120,7 @@ public Integer then(@NonNull Task<HttpsCallableResult> task) throws Exception {
 
     // [START function_add_message]
     private Task<String> addMessage(String text) {
-        // Create the arguments to the callable function, which is just one string
+        // Create the arguments to the callable function.
         Map<String, Object> data = new HashMap<>();
         data.put("text", text);
         data.put("push", true);