doc: change os x tag to macos

PR-URL: https://github.com/nodejs/node/pull/9009
Reviewed-By: Julien Gilli <jgilli@nodejs.org>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

doc/onboarding-extras.md

@@ -77,7 +77,7 @@ Please use these when possible / appropriate
 ### Other Labels
 
 * Operating system labels
-  * `os x`, `windows`, `smartos`, `aix`
+  * `macos`, `windows`, `smartos`, `aix`
   * No linux, linux is the implied default
 * Architecture labels
   * `arm`, `mips`, `s390`, `ppc`