fix test

Author: steveklabnik

src/rust-next/tryfrom-and-tryinto.md

@@ -14,5 +14,10 @@ arrays is tedious to do manually. With the new traits, it can be done inline
 with `.try_into()`:
 
 ```rust
+use std::convert::TryInto;
+# fn main() -> Result<(), Box<dyn std::error::Error>> {
+# let slice = &[1, 2, 3, 4][..];
 let num = u32::from_be_bytes(slice.try_into()?);
+# Ok(())
+# }
 ```