FHS says to put documents in /usr/share/doc

(Logical change 1.90)
diff --git a/Makefile b/Makefile
index 528cac0..fa820db 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 DESTDIR=
 SBINDIR=/usr/sbin
 CONFDIR=/etc/iproute2
-DOCDIR=/usr/doc/iproute2
+DOCDIR=/usr/share/doc/iproute2
 MANDIR=/usr/share/man
 
 # Path to db_185.h include