Fix 2013/hou/Makefile

After 2013/hou/doc/example.markdown was renamed to
2013/hou/doc/example.md to match the rest of the repo running 'make'
would fail because of the file referenced in the Makefile no longer
exists.

2013/hou/Makefile

@@ -110,7 +110,7 @@ PROG= ${ENTRY}
 #
 OBJ= ${PROG}.o
 DATA= luna.jpg old_default.jpg otherroom.jpg otherroom.scene \
-	doc/camera.png doc/cosine.png doc/draft.png doc/example.markdown
+	doc/camera.png doc/cosine.png doc/draft.png doc/example.md
 TARGET= ${PROG}
 #
 ALT_OBJ=