bump version ngpreview.5

Author: niko-ng

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "ng-rocksdb"
 description = "Rust wrapper for Facebook's RocksDB embeddable database, fork for Nextgraph.org"
-version = "0.21.0-ngalpha.5"
+version = "0.21.0-ngpreview.5"
 edition = "2018"
 rust-version = "1.60"
 authors = ["Tyler Neely <t@jujit.su>", "David Greenberg <dsg123456789@gmail.com>"]