Remove ` ignore```` from E0705 test

Author: CAD97

compiler/rustc_error_codes/src/error_codes/E0705.md

@@ -3,7 +3,7 @@ current edition, but not in all editions.
 
 Erroneous code example:
 
-```ignore (limited to a warning during 2018 edition development)
+```rust2018,compile_fail,E0705
 #![feature(rust_2018_preview)]
 #![feature(test_2018_feature)] // error: the feature
                                // `test_2018_feature` is