docs: Fix a few typos

[timgates42]

There are small typos in:

• pybacktest/ami_funcs.py
• pybacktest/backtest.py

Fixes:

• Should read occurred rather than occured.
• Should read mimicking rather than mimicing.
• Should read functions rather than funtions.
• Should read altogether rather than altogather.

Semi-automated pull request generated by https://github.com/timgates42/meticulous/blob/master/docs/NOTE.md