docs(okta): add how to use email in urns

[anshbansal]

Checklist

• The PR conforms to DataHub's Contributing Guideline (particularly Commit Message Format)
• Links to related issues (if applicable)
• Tests for the changes have been added/updated (if applicable)
• Docs related to the changes have been added/updated (if applicable). If a new feature has been added a Usage Guide has been added for the same.
• For any breaking change/potential downtime/deprecation/big changes an entry has been made in Updating DataHub

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.01 ⚠️

Comparison is base (f348113) 74.97% compared to head (0b7d5f8) 74.97%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7708      +/-   ##
==========================================
- Coverage   74.97%   74.97%   -0.01%     
==========================================
  Files         353      353              
  Lines       35406    35406              
==========================================
- Hits        26547    26545       -2     
- Misses       8859     8861       +2     

Flag	Coverage Δ
pytest-testIntegration	50.92% <ø> (-0.01%)	⬇️
pytest-testIntegrationBatch1	36.48% <ø> (ø)
pytest-testQuick	63.57% <ø> (ø)
pytest-testSlowIntegration	32.96% <ø> (ø)

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Impacted Files	Coverage Δ
...tion/src/datahub/ingestion/source/identity/okta.py	82.43% <ø> (ø)

... and 1 file with indirect coverage changes

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