[tracing] Fix JSON format to mention "spaces" instead of "pages".

As shown in the commit description of https://crrev.com/c/1619763, the JSON
format was supposed to refer to a list of "spaces" and not "pages", this was a
typo.

Bug: v8:9186
Change-Id: I1a674dac8af4b27b7ee46041e8c7a533bad8e68b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1657917Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#62139}

src/heap/heap.cc

@@ -547,7 +547,7 @@ void Heap::DumpJSONHeapStatistics(std::stringstream& stream) {
     MEMBER("malloced_memory") << stats.malloced_memory() << ","
     MEMBER("external_memory") << stats.external_memory() << ","
     MEMBER("peak_malloced_memory") << stats.peak_malloced_memory() << ","
-    MEMBER("pages") << LIST(
+    MEMBER("spaces") << LIST(
       SpaceStatistics(RO_SPACE)      << "," <<
       SpaceStatistics(NEW_SPACE)     << "," <<
       SpaceStatistics(OLD_SPACE)     << "," <<