Reformat the comment to match the rest of the file

Author: nkaretnikov

src/macros/dry.md

@@ -57,7 +57,7 @@ mod test {
         }
     }
 
-    // Test `add_assign`, `mul_assign` and `sub_assign`
+    // Test `add_assign`, `mul_assign`, and `sub_assign`.
     test!(add_assign, 1u32, 2u32, 3u32);
     test!(mul_assign, 2u32, 3u32, 6u32);
     test!(sub_assign, 3u32, 2u32, 1u32);