Tidy up source file and fix typo.

src/librustdoc/test.rs

@@ -428,7 +428,7 @@ pub fn make_test(s: &str,
 
             if !found_extern_crate {
                 if let ast::ItemKind::ExternCrate(original) = item.node {
-                    // This code will never be reached if `cratename` is none ecause
+                    // This code will never be reached if `cratename` is none because
                     // `found_extern_crate` is initialized to `true` if it is none.
                     let cratename = cratename.unwrap();