Create a **function**called `edit_distance` which calculates the minimum number of changes (insertions, deletions and/or substitutions) which need to be made to a string `source` to transform to another string `target`.
Create a **function**named `edit_distance`, which calculates the minimum number of changes (insertions, deletions and/or substitutions) which are needed to transform the `source` string to the `target` string.