docs: fix a few simple typos

[areski]

There are small typos in:

• nvd3/NVD3Chart.py
• nvd3/bulletChart.py
• nvd3/ipynb.py
• nvd3/multiChart.py

Fixes:

• Should read rendered rather than renderd.
• Should read overridden rather than overriden.
• Should read javascript rather than javscript.
• Should read gauge rather than guage.

Closes #170

[areski]

Thanks!