fix line lengths

Author: RalfJung

src/librustc_typeck/diagnostics.rs

@@ -1611,9 +1611,9 @@ static BAR: _ = "test"; // error, explicitly write out the type instead
 "##,
 
 E0122: r##"
-An attempt was made to add a generic constraint to a type alias. This constraint is
-entirely ignored. For backwards compatibility, Rust still allows this with a warning.
-Consider the example below:
+An attempt was made to add a generic constraint to a type alias. This constraint
+is entirely ignored. For backwards compatibility, Rust still allows this with a
+warning. Consider the example below:
 
 ```
 trait Foo{}