doc: fix markdown parsing on doc/api/os.md

PR-URL: #33067
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Rich Trott <[email protected]>

Author: juanarbol

doc/api/os.md

@@ -389,7 +389,7 @@ operating system response.
 
 Throws a [`SystemError`][] if a user has no `username` or `homedir`.
 
-## `os.version()`
+## `os.version()`
 <!-- YAML
 added: v13.11.0
 -->