doc: standardize on "host name" in http2.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>

doc/api/http2.md

@@ -2783,7 +2783,7 @@ console.log(request.headers);
 
 See [HTTP/2 Headers Object][].
 
-In HTTP/2, the request path, hostname, protocol, and method are represented as
+In HTTP/2, the request path, host name, protocol, and method are represented as
 special headers prefixed with the `:` character (e.g. `':path'`). These special
 headers will be included in the `request.headers` object. Care must be taken not
 to inadvertently modify these special headers or errors may occur. For instance,