doc: change 'unix' to 'Unix' in ninja guide

A recent PR converted all instances of 'unix' and 'UNIX' to 'Unix'
in the API docs. This commit fixes one instance in the guides in
preparation for a relevant lint rule.

PR-URL: #28619
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

doc/guides/building-node-with-ninja.md

@@ -2,7 +2,7 @@
 
 The purpose of this guide is to show how to build Node.js using [Ninja][], as
 doing so can be significantly quicker than using `make`. Please see
-[Ninja's site][Ninja] for installation instructions (unix only).
+[Ninja's site][Ninja] for installation instructions (Unix only).
 
 To build Node.js with ninja, there are 3 steps that must be taken: