clarify reboot comment to reflect reality

It seems the delay for the second rotation was changed without
changing the descriptive comment, in:

199a62a1 delay hosts and shutdown should not be the same for rotation hosts

It was also not clear which delay we were talking about here, adding
"every X" should clarify that.

howto/upgrades.md

@@ -177,10 +177,10 @@ currently can't unlock itself, naturally.
 This routine should be able to reboot all hosts with a `rebootPolicy`
 defined to `justdoit` or `rotation`:
 
-    echo "rebooting 'justdoit' hosts with a 10-minute delay...."
+    echo "rebooting 'justdoit' hosts with a 10-minute delay, every 30 minutes...."
     ./reboot -H $(ssh alberti.torproject.org 'ldapsearch -h db.torproject.org -x -ZZ -b ou=hosts,dc=torproject,dc=org -LLL "(rebootPolicy=justdoit)" hostname | awk "\$1 == \"hostname:\" {print \$2}" | sort -R') --delay-hosts=1800 --delay-shutdown=10 -v
 
-    echo "rebooting 'rotation' hosts with a 30-minute delay...."
+    echo "rebooting 'rotation' hosts with a 10-minute delay, every 30 minutes...."
     ./reboot -H $(ssh alberti.torproject.org 'ldapsearch -h db.torproject.org -x -ZZ -b ou=hosts,dc=torproject,dc=org -LLL "(rebootPolicy=rotation)" hostname | awk "\$1 == \"hostname:\" {print \$2}" | sort -R') --delay-hosts=1800 --delay-shutdown=10 -v
 
 ### Rebooting KVM hosts