doc: fix an SNI mistyped as SNS

PR-URL: https://github.com/nodejs/node/pull/9665
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

doc/api/tls.md

@@ -343,7 +343,7 @@ added: v0.5.3
   `cert`, `ca`, etc).
 
 The `server.addContext()` method adds a secure context that will be used if
-the client request's SNS hostname matches the supplied `hostname` (or wildcard).
+the client request's SNI hostname matches the supplied `hostname` (or wildcard).
 
 ### server.address()
 <!-- YAML