reference: follow idiom in code snippet

tshepang · tshepang · commit f34eafdcf948 · 2015-09-24T08:25:43.000+02:00
diff --git a/src/doc/reference.md b/src/doc/reference.md
@@ -921,7 +921,7 @@ An example of a function:
 
 ```
 fn add(x: i32, y: i32) -> i32 {
-    return x + y;
+    x + y
 }
 ```
 
