[docs] typo in GC API reference

The GarbageCollected<> template takes the class name as a parameter.

Change-Id: I118947d646c97c84534b218820af9b2c7e0a60db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2531840
Auto-Submit: Johann Koenig <johannkoenig@google.com>
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826749}

third_party/blink/renderer/platform/heap/BlinkGCAPIReference.md

@@ -239,7 +239,7 @@ It may take some time for the pointer in a `WeakMember<T>` to become `nullptr` a
 because this rewrite is only done within Blink GC's garbage collection period.
 
 ```c++
-class SomeGarbageCollectedClass : public GarbageCollected<GarbageCollectedSomething> {
+class SomeGarbageCollectedClass : public GarbageCollected<SomeGarbageCollectedClass> {
   ...
 private:
   Member<AnotherGarbageCollectedClass> another_; // OK, retained by Member<T>.