docs: Fix a few typos

[timgates42]

There are small typos in:

• doc/index.rst
• doc/tutorial.rst
• examples/warm_start_als.py
• examples/warm_start_mcmc.py
• fastFM/bpr.py
• fastFM/sgd.py
• fastFM/tests/test_als.py
• fastFM/validation.py

Fixes:

• Should read tuning rather than tunning.
• Should read initialize rather than initalize.
• Should read representation rather than reprensentation.
• Should read refer rather than refere.
• Should read continuous rather than continous.

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