Makefile: write the manpage to the right file and use the processed markdown document

Makefile

@@ -56,7 +56,7 @@ README.txt: README.md
 
 youtube-dl.1: README.md
 	python devscripts/prepare_manpage.py >youtube-dl.1.temp.md
-	pandoc -s -f markdown -t man README.md -o youtube-dl.1.temp.md
+	pandoc -s -f markdown -t man youtube-dl.1.temp.md -o youtube-dl.1
 	rm -f youtube-dl.1.temp.md
 
 youtube-dl.bash-completion: youtube_dl/*.py youtube_dl/*/*.py devscripts/bash-completion.in