docs: Add missed configuration option (sqlc-dev#2188)

Author: abekoh

docs/reference/config.md

@@ -213,6 +213,7 @@ sql:
             import: "a/b/v2"
             package: "b"
             type: "MyType"
+            pointer: true
 ```
 
 When generating code, entries using the `column` key will always have preference over