Trailing slashes on volumes fail on newer versions of docker-compose (#949)

* Trailing slashes on volumes fail on newer versions of docker-compose

See [Issue 947](https://github.com/pi-hole/docker-pi-hole/issues/947)
for the background.

This PR proposes removing trailing slashes from all documentation,
examples and scripts.

Signed-off-by: Phill Kelley <pmk.57t49@lgosys.com>

docker-compose.yml.example

@@ -17,8 +17,8 @@ services:
       # WEBPASSWORD: 'set a secure password here or it will be random'
     # Volumes store your data between container upgrades
     volumes:
-      - './etc-pihole/:/etc/pihole/'
-      - './etc-dnsmasq.d/:/etc/dnsmasq.d/'
+      - './etc-pihole:/etc/pihole'
+      - './etc-dnsmasq.d:/etc/dnsmasq.d'
       # run `touch ./var-log/pihole.log` first unless you like errors
       # - './var-log/pihole.log:/var/log/pihole.log'
     # Recommended but not required (DHCP needs NET_ADMIN)