Update instructions for migration

Today when I upgraded, I found that I had to also move/delete the data dir (not only the rethinkdb_data dir, as the readme says) in my instance folder. Updated the description to reflect this.

Here is a log that shows it:
```
$ sudo /etc/init.d/rethinkdb stop
$ mv rethinkdb_data/ bu_rd_data
```
....SNIP UPGRADE SNIP...
```
$ sudo /etc/init.d/rethinkdb start
rethinkdb: logger: Starting instance. (logging to `/var/lib/rethinkdb/logger/data/log_file')
```
-- Checked admin UI. Not responding.
```
/var/lib/rethinkdb/logger$ more data/log_file
.......SNIP
2013-06-14T12:25:50.237999367 19958.094500s info: Storage engine shut down.
2013-06-14T12:26:36.428862914 0.010244s info: Running rethinkdb 1.6.0-0ubuntu1~precise (GCC 4.6.3)...
2013-06-14T12:26:36.435715970 0.017096s info: Running on Linux 3.2.0-40-virtual x86_64
2013-06-14T12:26:36.435775856 0.017156s info: Loading data from directory /var/lib/rethinkdb/logger/data
2013-06-14T12:26:36.452642142 0.034023s error: File version is incorrect. This file was created with RethinkDB's serializer version 1.5, but you are trying to read it with version 1.6.
2013-06-14T12:27:21.795479013 0.009312s info: Running rethinkdb 1.6.0-0ubuntu1~precise (GCC 4.6.3)...
2013-06-14T12:27:21.799406545 0.013239s info: Running on Linux 3.2.0-40-virtual x86_64
2013-06-14T12:27:21.799459035 0.013292s info: Loading data from directory /var/lib/rethinkdb/logger/data
2013-06-14T12:27:21.804126474 0.017959s error: File version is incorrect. This file was created with RethinkDB's serializer version 1.5, but you are trying to read it with version 1.6.
$ sudo mv data bu_data
$ sudo /etc/init.d/rethinkdb start
```
-- Web interface responding. Log file all good.

Although the solution was simple, especially after reading the log, I think the README should be correct as well. Hence this pull request.

Great work on 1.6 btw!