Issue #323: Followup. Fixing misnamed function to be admin_menu instead of admin_bar.

Followup to the PR at https://github.com/backdrop/backdrop/pull/474.

Fixes https://github.com/backdrop/backdrop-issues/issues/323.

This just addresses a typo where we renamed "system_admin_menu_block_page" to "system_admin_bar_block_page" accidentally.