Update tailscale_vpn.md (an RFC)

- an RFC, similar to the wording on the following site:
 https://www.rfc-editor.org/faq/

Author: SergeCroise

docs/guides/security/tailscale_vpn.md

@@ -69,7 +69,7 @@ Once your server is authenticated with Tailscale, it will get a Tailscale IPv4 a
 tailscale ip -4
 ```
 
-It will also get a RFC 4193 (Unique Local Address) Tailscale IPv6 address:
+It will also get an RFC 4193 (Unique Local Address) Tailscale IPv6 address:
 
 ```bash
 tailscale ip -6