doc: standardize on "host name" in os.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/os.md

@@ -175,7 +175,7 @@ added: v0.3.3
 
 * Returns: {string}
 
-Returns the hostname of the operating system as a string.
+Returns the host name of the operating system as a string.
 
 ## `os.loadavg()`
 <!-- YAML