Fix the directive to hide documentation in the summarize_diff module.

The existing directive was an attribute on the first element of the module rather than on the module itself. So it had no effect.

PiperOrigin-RevId: 549346674

Author: hovinenb

googletest/src/matcher_support/summarize_diff.rs

@@ -12,7 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#[doc(hidden)]
+#![doc(hidden)]
+
 use std::borrow::Cow;
 use std::fmt::{Display, Write};