devel/cvsweb: add mkdir /var/www/dev to README script

Hi,

cvsweb touts that: "You can run this README directly with sh(1)", but
/var/www/dev isn't present on systems by default.

This (admittedly trivial) diff adds dev to the mkdir command. Then passing the
README to sh(1) succeeds.

Interest in adding this?

Index: pkg/README
===================================================================
RCS file: /cvs/ports/devel/cvsweb/pkg/README,v
retrieving revision 1.14
diff -u -p -r1.14 README
--- pkg/README 1 Mar 2017 11:56:05 -0000 1.14
+++ pkg/README 5 Jun 2018 06:13:25 -0000
@@ -15,7 +15,7 @@
# First, create the basic directory structure:

cd /var/www
-mkdir -p tmp usr
+mkdir -p tmp usr dev
chown www:www tmp # needs to be writeable for the www user

cd /var/www/usr