update namespace of Background in the documentation (docs/extraction_quickstart.rst)

[rudolffu]

Replace specreduce.tracing.Background with specreduce.background.Background for correct calling of the Background submodule.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 82.16%. Comparing base (408547c) to head (c0bad8c).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #215 +/- ## ======================================= Coverage 82.16% 82.16% ======================================= Files 10 10 Lines 1015 1015 ======================================= Hits 834 834 Misses 181 181 ```

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