mkone: fix man target (thanks Amavect)

Amavect wrote:
> mkone and mkmany have backwards targets for installing man pages.
> This patch makes 'mk man' actually work for mkfiles that include mkone.
> mkmany is not easily fixed without breaking changes.
> It may go without saying that external repos should write their own mkfiles.

sys/src/cmd/mkone

@@ -56,8 +56,7 @@ safeinstallall:V:
 %.acid: %.$O $HFILES
 	$CC $CFLAGS -a $stem.c >$target
 
-%.man:	$MAN/$stem
-	cp $stem.man $MAN/$stem
-
-man:V:	$TARG.man
+$MAN/%:    %.man
+	cp $prereq $target
 
+man:V:    $MAN/$TARG