DOC: greater consistency and spell-check for intro docs
I read through introductory docs, and made the following changes:

- Added missing periods, which were sometimes absent in short sentences.
- In other parts of the documentation, code examples appear to typically be introduced with a colon. This has been added in some sentences which introduce code and do not end in a period nor a colon.
- Updated a reference to the Python 2 docs to Python 3.
- Restructured a few sentences slightly.
- Found an example of code that did not run due to an extra `}`, which I removed.
