Fix a few typos in docstrings and comments.

Hi Olga,
I'm so glad I stumbled across this library! It captures a number of things I used to do manually, too. This is the first time I'm using github and pull requests for collaboration, so I thought I'd start with a simple, non-controversial commit - a few typos I spotted while looking through the sourcecode. Furthermore, I noticed a couple of comments which seem to break off halfway through a sentence, but as I didn't know how they were meant to end, I didn't change them. Here they are (all under prettyplotlib/):

_pcolormesh.py:44 # If
_scatter.py:19 # [...] Can specify
_scatter.py:23 # [...] You can assign the