modify repeat_entry.rb example

fixes #87 

Just an example change to better illustrate usage of gather with a hash as it wasn't obvious to me to use erb to access the @key in the question (unfamiliarity with highline).

Prior to this change, the example would ask "Enter your password:  " twice.
Now uses hash entries as the question and asks "Enter a password:  " followed by "Please type it again for verification:   "