cppgc: Fix typo in docs

Bug: chromium:1056170
Change-Id: I0854b9b144ad47dc7ea0b16862fea1583aec7402
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2190416
Auto-Submit: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67676}

include/cppgc/custom-space.h

@@ -34,7 +34,7 @@ class CustomSpaceBase {
  * };
  * class CustomSpace2 : public CustomSpace<CustomSpace2> {
  *  public:
- *   static constexpr CustomSpaceIndex kSpaceIndex = 2;
+ *   static constexpr CustomSpaceIndex kSpaceIndex = 1;
  * };
  * \endcode
  */