Update Solution argument names to match prompt

[pyamin1878]

Here is the function signature of the prompt:

def multi_word_search(doc_list, keywords):

Here is the function when you print out the solution:

def multi_word_search(documents, keywords):
    keyword_to_indices = {}
    for keyword in keywords:
        keyword_to_indices[keyword] = word_search(documents, keyword)
    return keyword_to_indices

This pr will make the function consistent with the provided exercise.

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.