Fix bug in week2-NER.ipynb that misses last tweet when reading data.

[ca-scribner]

Previous code misses last tweet unless file ends with two empty lines (a single empty line is interpreted by the for loop as the end of the file and thus is not passed to the code at all, which means we never reach the "if not line" inside the loop for the last tweet). Use a for/else to catch the last tweet.

With block added to implicitly close file. Not necessary to fix above bug