(clean): Fix deletion of c.html

Makefile

@@ -21,6 +21,7 @@ c.txt: c.texi cpp.texi fp.texi fdl.texi
 	makeinfo --plaintext c.texi > $@
 
 clean:
-	rm -f c-manual.tgz c.dvi c.pdf c c-* c.doc c.html c.txt
+	rm -f c-manual.tgz c.dvi c.pdf c c-* c.doc c.txt
+	rm -rf c.html
 	rm -f c.aux c.cp c.fn c.ky c.log c.pg c.tmp c.toc c.tp c.vr
 	rm -f c.cps c.fns