corrected error

Makefile

@@ -20,7 +20,7 @@ HTMLOPTIONS += --mathjax=MathJax.js
 
 CLASS_SOURCES := $(sort $(filter-out README.md, $(wildcard *.md)))
 SOURCES := $(filter-out 00_macros.md, $(CLASS_SOURCES))
-SOURCES := $(filter-out 08_notes.md, $(CLASS_SOURCES))
+SOURCES := $(filter-out 08_notes.md, $(SOURCES))
 
 all:  cours.pdf cours.html