docs: update example in readme

[ainghazal]

The current example snippet in the README needed some work to catch up with the latest code changes.
This commit updates the usage shown in the example to create a namespace, and passes the expected context to the goipam.New constructor.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 46.71%. Comparing base (6d48337) to head (7cd2781).
Report is 2 commits behind head on master.

❗ Current head 7cd2781 differs from pull request most recent head 95d90cb

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Additional details and impacted files

@@             Coverage Diff             @@
##           master     #150       +/-   ##
===========================================
- Coverage   63.03%   46.71%   -16.33%     
===========================================
  Files          11       16        +5     
  Lines        1672     3427     +1755     
===========================================
+ Hits         1054     1601      +547     
- Misses        398     1596     +1198     
- Partials      220      230       +10     

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