Fix diagnostic_list error

Author: varkor

src/libsyntax/diagnostic_list.rs

@@ -380,7 +380,7 @@ the current edition.
 
 Erroneous code example:
 
-```compile_fail,E0705
+```ignore (limited to a warning during 2018 edition development)
 #![feature(rust_2018_preview)]
 #![feature(raw_identifiers)] // error: the feature `raw_identifiers` is
                              // included in the Rust 2018 edition