Fix contrib target

Makefile

@@ -4,11 +4,11 @@ all: yadm.md contrib
 yadm.md: yadm.1
 	@groff -man -Tascii ./yadm.1 | col -bx | sed 's/^[A-Z]/## &/g' | sed '/yadm(1)/d' > yadm.md
 
-CONTRIBUTORS:
+.PHONY: contrib
+contrib:
+	@echo "CONTRIBUTORS\n" > CONTRIBUTORS
 	@git shortlog -ns master gh-pages dev dev-pages | cut -f2 >> CONTRIBUTORS
 
-contrib: CONTRIBUTORS
-
 .PHONY: pdf
 pdf:
 	@groff -man -Tps ./yadm.1 > yadm.ps