Fix pasto that caused 'make distribution' to break when installing ppp.

src/etc/Makefile

@@ -78,7 +78,6 @@ distribution: distrib-dirs
 		install -c -o root -g wheel -m 644 options.leaf \
 		    ${DESTDIR}/etc/ppp; \
 		install -c -o root -g wheel -m 644 pap-secrets \
-		    ${DESTDIR}/etc/ppp; \
 		    ${DESTDIR}/etc/ppp)
 	cd namedb; install -c -o ${BINOWN} -g ${BINGRP} -m 644 ${NAMEDB} \
 	    ${DESTDIR}/etc/namedb