src: add a missing space in node_os.cc

This commit makes a small stylistic fix adding a missing space in
GetOSType function in node_os.cc

PR-URL: https://github.com/nodejs/node/pull/10931


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michal Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>