zbm-builder.sh: Fix links to documentation

The output of `./zbm-builder.sh -h` includes a link to
<https://github.com/zbm-dev/zfsbootmenu/blob/master/docs/BUILD.md>,
which hasn't existed since 31fc472.

This PR replaces both documentation links with a single link to
<https://docs.zfsbootmenu.org/>.

Author: noelleleigh

zbm-builder.sh

@@ -86,8 +86,7 @@ OPTIONS:
 
 For more information, see documentation at
 
-  https://github.com/zbm-dev/zfsbootmenu/blob/master/README.md
-  https://github.com/zbm-dev/zfsbootmenu/blob/master/docs/BUILD.md
+  https://docs.zfsbootmenu.org/
 EOF
 }