Fix documentation for Vault CLI option (#828)

Author: hver

content/vault/v1.20.x/content/partials/global-settings/both/client-cert.mdx

@@ -4,7 +4,7 @@
 
 Path to a PEM-encoded CA certificate file on the local disk. Used for TLS
 communication with the server. **The specified certificate must match to the
-private key specified with `-client-cert`**.
+private key specified with `-client-key`**.
 
 **Examples**: