Change wording, Add example for unsupported trailing vertical bar

Author: alexandrebodin

spec/Section 3 -- Type System.md

@@ -763,15 +763,19 @@ Union types are defined by delimiting one or more types with a single vertical b
 union SearchResult = Photo | Person
 ```
 
-You may also use a leading vertical bar.
+You may also use a leading vertical bar:
 
 ```graphql
 union SearchResult =
   | Photo
   | Person
 ```
 
-Note Trailing vertical bars are not supported.
+Trailing delimiters are not supported:
+
+```!graphql
+union SearchResult = Photo | Person |
+```
 
 ### Enums