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

R=yzshen@chromium.org

Review-Url: https://codereview.chromium.org/2851953003
Cr-Commit-Position: refs/heads/master@{#468256}

mojo/public/tools/bindings/README.md

@@ -267,7 +267,7 @@ 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;