Remove § about prototype_name customization in 2.0

I don't think this was even possible in 2.0

UPGRADE-2.1.md

@@ -423,19 +423,10 @@
   * In the collection type's template, the default name of the prototype field
     has changed from `$$name$$` to `__name__`.
 
-    For custom names, dollar signs are no longer prepended and appended. You are
-    advised to prepend and append two underscores wherever you specify a value
-    for the field's "prototype_name" option.
-
-    Before:
-
-    ```
-    $builder->add('tags', 'collection', array('prototype' => 'proto'));
-
-    // results in the name "$$proto$$" in the template
-    ```
-
-    After:
+    You can now customize the name of the prototype field by changin the
+    "prototype_name" option. You are advised to prepend and append two
+    underscores wherever you specify a value for the field's "prototype_name"
+    option.
 
     ```
     $builder->add('tags', 'collection', array('prototype_name' => '__proto__'));