Fix bug in movie lookup code

When adding a movie to the movie queue, the `parse_what` function that
interprets the search term will currently never perform a search using the
imdb_id or tmdb_id.

Currently, when populating the `search_entry` `Entry` dict, the code uses the
string `'field'` instead of the value of the variable `field` to check for id
attributes of the result, causing the `search_entry` dict to only use the
title of the movie and disregard the imdb/tvdb id.

This patch fixes the issue by replacing the string `'field'` with the variable
`field`.