fix the disk backed array example

As recommened by Mark W. on:

https://groups.google.com/a/continuum.io/forum/#!topic/blaze-dev/aflz5f7-efc
add the '$' where missing in the README
Merge pull request #53 from esc/consistent_shell_prompt

add the '$' where missing in the README
Merge pull request #52 from esc/fix_disk_backed_example

fix the disk backed array example