Fixed EXPERIMENTAL_BUCKETLIST_DB_INDEX_PAGE_SIZE_EXPONENT (stellar#4378)

# Description

Resolves stellar#4377

# Checklist
- [x] Reviewed the
[contributing](https://github.com/stellar/stellar-core/blob/master/CONTRIBUTING.md#submitting-changes)
document
- [x] Rebased on top of master (no merge commits)
- [x] Ran `clang-format` v8.0.0 (via `make format` or the Visual Studio
extension)
- [x] Compiles
- [x] Ran all tests
- [ ] If change impacts performance, include supporting evidence per the
[performance
document](https://github.com/stellar/stellar-core/blob/master/performance-eval/performance-eval.md)

src/main/Config.cpp

@@ -1099,7 +1099,7 @@ Config::processConfig(std::shared_ptr<cpptoml::table> t)
             else if (item.first ==
                      "EXPERIMENTAL_BUCKETLIST_DB_INDEX_PAGE_SIZE_EXPONENT")
             {
-                BUCKETLIST_DB_INDEX_CUTOFF = readInt<size_t>(item);
+                BUCKETLIST_DB_INDEX_PAGE_SIZE_EXPONENT = readInt<size_t>(item);
                 CLOG_WARNING(
                     Bucket,
                     "EXPERIMENTAL_BUCKETLIST_DB_INDEX_PAGE_SIZE_EXPONENT is "