Fix overlong line length

Author: notriddle

src/librustc/diagnostics.rs

@@ -1647,8 +1647,8 @@ the size of the provided type. Such an enum can be cast to a value of the same
 type as well. In short, `#[repr(u8)]` makes the enum behave like an integer
 with a constrained set of allowed values.
 
-Only field-less enums can be cast to numerical primitives, so this attribute will
-not apply to structs.
+Only field-less enums can be cast to numerical primitives, so this attribute
+will not apply to structs.
 
 `#[repr(packed)]` reduces padding to make the struct size smaller. The
 representation of enums isn't strictly defined in Rust, and this attribute