Makefile: fix css path

Makefile

 all: public/index.html \
+	public/github-pandoc.css \
 	public/1_understanding_a_computer.html \
 	public/2_using_the_ifb_cloud.html \
 	public/3_first_steps_in_a_terminal.html \
@@ -12,6 +13,8 @@ all: public/index.html \
 	public/11_install_system_programs.html \
 	public/12_virtualization.html
 
+public/github-pandoc.css: github-pandoc.css
+	cp github-pandoc.css public/github-pandoc.css
 
 public/index.html: index.md github-pandoc.css
 	pandoc -s -c github-pandoc.css index.md -o public/index.html