Traits where syntax's extra usage example more clearly

r? @steveklabnik

Author: wuranbo

src/doc/book/traits.md

@@ -397,10 +397,10 @@ fn normal<T: ConvertTo<i64>>(x: &T) -> i64 {
 }
 
 // can be called with T == i64
-fn inverse<T>() -> T
+fn inverse<T>(x: i32) -> T
         // this is using ConvertTo as if it were "ConvertTo<i64>"
         where i32: ConvertTo<T> {
-    42.convert()
+    x.convert()
 }
 ```