Change 'enum' -> 'union' in tagged union example.

mojo/public/tools/bindings/README.md

 # Mojom IDL and Bindings Generator
 This document is a subset of the [Mojo documentation](/mojo).
 
 [TOC]
 
 ## Overview
 
 Mojom is the IDL for Mojo bindings interfaces. Given a `.mojom` file, the
 [bindings generator](https://cs.chromium.org/chromium/src/mojo/public/tools/bindings)
 outputs bindings for all supported languages: **C++**, **JavaScript**, and
@@ skipping 249 matching lines @@
 
 Mojom supports tagged unions using the **union** keyword. A union is a
 collection of fields which may taken the value of any single one of those fields
 at a time. Thus they provide a way to represent a variant value type while
 minimizing storage requirements.
 
 Union fields may be of any type supported by [struct](#Structs) fields. For
 example:
 
 ```cpp
-enum ExampleUnion {
+union ExampleUnion {
   string str;
   StringPair pair;
   int64 id;
   array<uint64, 2> guid;
   SampleInterface iface;
 };
 ```
 
 For details on how unions like this translate to generated bindings code, see
 [documentation for individual target languages](#Generated-Code-For-Target-Languages).
@@ skipping 482 matching lines @@
 
 ## Additional Documentation
 
 [Mojom Message Format](https://docs.google.com/document/d/13pv9cFh5YKuBggDBQ1-AL8VReF-IYpFOFpRfvWFrwio/edit)
 :    Describes the wire format used by Mojo bindings interfaces over message
      pipes.
 
 [Input Format of Mojom Message Validation Tests](https://docs.google.com/document/d/1-y-2IYctyX2NPaLxJjpJfzVNWCC2SR2MJAD9MpIytHQ/edit)
 :    Describes a text format used to facilitate bindings message validation
      tests.