doc: standardize on "host name" in https.md

Our docs have a mix of "hostname" and "host name" in prose.

Let's follow the usage of Unix man pages, RFCs, and most
professionally-edited sources, and use "host name" in prose and
"hostname" to refer to the command and in code.

Lint rule forthcoming.

PR-URL: #31326
Refs: #31073
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

Author: Trott

doc/api/https.md

@@ -39,7 +39,7 @@ changes:
   * `servername` {string} the value of
     [Server Name Indication extension][sni wiki] to be sent to the server. Use
     empty string `''` to disable sending the extension.
-    **Default:** hostname of the target server, unless the target server
+    **Default:** host name of the target server, unless the target server
     is specified using an IP address, in which case the default is `''` (no
     extension).