Add xref to safe modes page

Author: ggrossetie

docs/modules/cli/pages/options.adoc

@@ -8,7 +8,7 @@
   Can be used as a way to chroot the execution of the program.
 
 *-S, --safe-mode*=_SAFE_MODE_::
-  Set safe mode level: _unsafe_, _safe_, _server_ or _secure_.
+  Set xref:asciidoctor::safe-modes.adoc[safe mode] level: _unsafe_, _safe_, _server_ or _secure_.
   Disables potentially dangerous macros in source files, such as `include::[]`.
   If not set, the safe mode level defaults to _unsafe_ when Asciidoctor is invoked using this script.