docs: update recommended `dinghy create` options
docs: update recommended `dinghy create` options

The docs now recommend giving dinghy 50GB of disk space instead of the
default 20GB. Hopefully this will help alleviate annoyances caused by
out-of-space errors when you are developing multiple dockerized services

Before this commit, doing a fresh install of canvas used up 38% of
dinghy's available disk space

Change-Id: Ifbbcd2aaee073b7beb9a7c724c4cec1a0ae7ac4f
Reviewed-on: https://gerrit.instructure.com/103873
Tested-by: Jenkins
Reviewed-by: Bryan Petty <33564f6eff3d808d3eaaa056a1631fdcde5faed6@instructure.com>
Product-Review: Andrew Huff <b334cc62d1472ff334a155064665c29200a80f59@instructure.com>
QA-Review: Andrew Huff <b334cc62d1472ff334a155064665c29200a80f59@instructure.com>

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