[rocksdb] Fix typo in enabling zstd feature

[BradBarnich]

Thanks @kobykahane for catching...

follow up to #7304

[BradBarnich]

I think the build failure was from a github hiccup:

Building package rocksdb[core,zlib]:x64-osx...
Could not locate cached archive: /Users/vagrant/azure_agent/_work/2/s/archives/16/16cac0c5c1f98c7ed7a98c45bb922fced7543432.zip
-- Downloading https://github.com/facebook/rocksdb/archive/v6.1.2.tar.gz...
-- Downloading https://github.com/facebook/rocksdb/archive/v6.1.2.tar.gz... Failed. Status: 22;"HTTP response code said error"
CMake Error at scripts/cmake/vcpkg_download_distfile.cmake:172 (message):
      
      Failed to download file.
      If you use a proxy, please set the HTTPS_PROXY and HTTP_PROXY environment
      variables to "https://user:password@your-proxy-ip-address:port/".
      Otherwise, please submit an issue at https://github.com/Microsoft/vcpkg/issues

[cbezault]

Resolved by #7558