removal of md5 from example code

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/10/static/citext.html
Description:

The documentation at
https://www.postgresql.org/docs/current/static/citext.html shows an example
using md5 for password hashes. This is generally a bad practice and not
relevant to the feature documented.

I recommend removing the password column from this example or replacing the
md5 hash with something more secure (a secure hash algorithm with a salt).