Correct the code comment in Compaction::KeyNotExistsBeyondOutputLevel (#4902)

Summary:
Even one key falls in a file's range, we can not infer it definitely exists in this file.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4902

Differential Revision: D13795018

Pulled By: siying

fbshipit-source-id: 590956f727e9440fcdee55ad9541ace934c64914

db/compaction.cc

@@ -373,8 +373,8 @@ bool Compaction::KeyNotExistsBeyondOutputLevel(
         if (user_cmp->Compare(user_key, f->largest.user_key()) <= 0) {
           // We've advanced far enough
           if (user_cmp->Compare(user_key, f->smallest.user_key()) >= 0) {
-            // Key falls in this file's range, so definitely
-            // exists beyond output level
+            // Key falls in this file's range, so it may
+            // exist beyond output level
             return false;
           }
           break;