etc/examples/doas.conf add persist for group wheel

[Horia Racoviceanu]

- for convenience, and perhaps best practice

Index: doas.conf
===================================================================
RCS file: /cvs/src/etc/examples/doas.conf,v
retrieving revision 1.1
diff -u -p -r1.1 doas.conf
--- doas.conf   3 Sep 2016 11:58:32 -0000       1.1
+++ doas.conf   13 Jun 2019 13:57:38 -0000
@@ -11,4 +11,4 @@
 #    SUBPACKAGE WRKOBJDIR SUDO_PORT_V1 } :wsrc
 
 # Allow wheel by default
-permit keepenv :wheel
+permit persist keepenv :wheel