Code in intro, minor improvement

[wstomv]

In https://github.com/csev/py4e/blob/master/book3/01-intro.mkd, I would like to suggest to improve readability by adding a space after the colon in:

python words.py
Enter file:words.txt
to 16

I.e., replace it by

python words.py
Enter file: words.txt
to 16