Fix trailing comma in head doctest

packages/core-macro/tests/rsx/trailing-comma-0.stderr

@@ -1,6 +1,7 @@
 error: Attributes must be separated by commas
-       = help: Did you forget a comma?
  --> tests/rsx/trailing-comma-0.rs:9:13
   |
 9 |             class: "foo bar"
   |             ^^^^^
+  |
+  = help: Did you forget a comma?

packages/html/src/document/head.rs

@@ -324,7 +324,7 @@ impl StyleProps {
 ///                 body {{
 ///                     background-color: red;
 ///                 }}
-///             "#,
+///             "#
 ///         }
 ///     }
 /// }