docs: Update sessions.md to reflect correct import behavior as described in session class docstring

[everettVT]

Looks like the import behavior is actually from daft.session import session

Changes Made

Update Sessions docs to use correct import behavior

Related Issues

No Issue

Checklist

• Documented in API Docs (if applicable)
• Documented in User Guide (if applicable)
• If adding a new documentation page, doc is added to docs/mkdocs.yml navigation
• Documentation builds and is formatted properly (tag @/ccmao1130 for docs review)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.26%. Comparing base (d7fd095) to head (bf59f3c).
Report is 58 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4381      +/-   ##
==========================================
- Coverage   77.54%   75.26%   -2.28%     
==========================================
  Files         841      839       -2     
  Lines      111643   114533    +2890     
==========================================
- Hits        86569    86208     -361     
- Misses      25074    28325    +3251     

see 74 files with indirect coverage changes

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[rchowell]

Thanks for the contribution!