Point at the proper supported version of Syntex

tessgriffin · tessgriffin · commit 42296b3fbe6c · 2016-01-16T22:47:41.000-07:00
0.22.0 is no longer supported by Diesel.
diff --git a/diesel_codegen/README.md b/diesel_codegen/README.md
@@ -34,7 +34,7 @@ build-dependencies.
 
 ```toml
 diesel_codegen = "^0.4.0"
-syntex = "^0.22.0"
+syntex = "^0.25.0"
 ```
 
 You'll need to move any code using annotations into a different file.
