Fix Supplement code sample.

Change-Id: I28900d4ca5da0d1683828fb82fdb12e88386a1cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2617478
Commit-Queue: Tal Pressman <talp@google.com>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Tal Pressman <talp@google.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#841365}

third_party/blink/renderer/platform/supplementable.h

@@ -77,7 +77,7 @@ namespace blink {
 //      public:
 //       static const char kSupplementName[];
 //
-//       NavigatorFoo& From(Navigator&);
+//       static NavigatorFoo& From(Navigator&);
 //     }
 //
 //     // static