docs: Fix a few typos

[timgates42]

There are small typos in:

• src/rinoh/backend/pdf/filter.py
• src/rinoh/backend/pdf/xobject/purepng.py
• src/rinoh/layout.py
• src/rinoh/paragraph.py
• src/rinoh/util.py

Fixes:

• Should read position rather than positon.
• Should read parsing rather than parsinng.
• Should read length rather than lenght.
• Should read interface rather than interace.
• Should read descriptor rather than desciptor.
• Should read container rather than contianer.
• Should read compression rather than compession.

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

[CLAassistant]

All committers have signed the CLA.

[brechtm]

Thanks, @timgates42!