Update session03.rst

- Grammatical typo
- Line 192, slide 12, is very long, not sure if the font can be shrunk
- Line 460, wrapped so it will fit on screen
- Line 835: assignment does not output, need to type the name explicitly
- Line 1169: typo, hexadecimal, not hexidecimal
- Line 1217: json.dumps produces output
Merge pull request #45 from fulv/patch-4

Update session03.rst

With thanks, @fulv