Fix volume paths in docker readme

While trying the examples in the readme I've noticed the following errors:
```
docker: Error response from daemon: create ~/opengrok-src/: "~/opengrok-src/" includes invalid characters for a local volume name, only "[a-zA-Z0-9][a-zA-Z0-9_.-]" are allowed. If you intended to pass a host directory, use absolute path.
```
and
```
docker: Error response from daemon: invalid volume specification: '/Users/ahornace/opengrok-data/:opengrok/data': invalid mount config for type "bind": invalid mount path: 'opengrok/data' mount path must be absolute.
```
Thanks.

Author: ahornace

docker/README.md

@@ -106,7 +106,7 @@ services:
     volumes:
        - '~/opengrok-src/:/opengrok/src/'  # source code
        - '~/opengrok-etc/:/opengrok/etc/'  # folder contains configuration.xml
-       - '~/opengrok-data/:opengrok/data'  # index and other things for source code
+       - '~/opengrok-data/:/opengrok/data/'  # index and other things for source code
 ```
 
 Save the file into `docker-compose.yml` and then simply run
@@ -120,9 +120,9 @@ docker run -d \
     --name opengrok \
     -p 8080:8080/tcp \
     -e REINDEX="60" \
-    -v "~/opengrok-src/:/opengrok/src/" \
-    -v "~/opengrok-etc/:/opengrok/etc/" \
-    -v "~/opengrok-data/:opengrok/data" \
+    -v ~/opengrok-src/:/opengrok/src/ \
+    -v ~/opengrok-etc/:/opengrok/etc/ \
+    -v ~/opengrok-data/:/opengrok/data/ \
     opengrok/docker:latest
 ```