Fix a copy-paste bug related to background threads in db_stress (#9485)

Summary:
Fixes a typo introduced in https://github.com/facebook/rocksdb/pull/9466.

Fixes https://github.com/facebook/rocksdb/issues/9482

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9485

Test Plan:
```
COMPILE_WITH_TSAN=1 make db_stress -j24
./db_stress --ops_per_thread=1000 --reopen=5
```

Reviewed By: ajkr

Differential Revision: D33928601

Pulled By: ltamasi

fbshipit-source-id: 3e01a0ca5fffb56c268c811cbe045413b225059a

db_stress_tool/db_stress_driver.cc

@@ -79,7 +79,7 @@ bool RunStressTest(StressTest* stress) {
     shared.IncBgThreads();
   }
 
-  if (FLAGS_compaction_thread_pool_adjust_interval > 0) {
+  if (FLAGS_continuous_verification_interval > 0) {
     shared.IncBgThreads();
   }