docs: Fix a few typos

[timgates42]

There are small typos in:

• feedgen/ext/dc.py
• feedgen/ext/geo_entry.py
• feedgen/feed.py
• feedgen/util.py

Fixes:

• Should read default rather than deault.
• Should read already rather than alredy.
• Should read omitted rather than omittet.
• Should read sure rather than shure.
• Should read rights rather than rightss.
• Should read generated rather than generaated.

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