docs: Update libraries.mdx (#906)

Add vaultx lib for Elixir client.

Author: iFleey

content/vault/v1.20.x/content/api-docs/libraries.mdx

@@ -81,6 +81,7 @@ $ Install-Package Vault
 
 - [libvault](https://hex.pm/packages/libvault)
 - [vaultex](https://hex.pm/packages/vaultex)
+- [vaultx](https://hex.pm/packages/vaultx)
 
 ### Erlang