took comment out of code block

no reason for a long comment in a code block when we could take it out, especially since it looks like it's using markdown (`struct`, `&` and `lvl`).

Author: jonmarkprice

src/doc/book/lifetimes.md

@@ -335,11 +335,13 @@ fn print<'a>(s: &'a str); // expanded
 
 fn debug(lvl: u32, s: &str); // elided
 fn debug<'a>(lvl: u32, s: &'a str); // expanded
+```
 
-// In the preceding example, `lvl` doesn’t need a lifetime because it’s not a
-// reference (`&`). Only things relating to references (such as a `struct`
-// which contains a reference) need lifetimes.
+In the preceding example, `lvl` doesn’t need a lifetime because it’s not a
+reference (`&`). Only things relating to references (such as a `struct`
+which contains a reference) need lifetimes.
 
+```rust,ignore
 fn substr(s: &str, until: u32) -> &str; // elided
 fn substr<'a>(s: &'a str, until: u32) -> &'a str; // expanded