docs: fix typos and improve error message in DateTimeFormat

[Nakshatra480]

Summary

This PR addresses small documentation and usability issues in the Intl.DateTimeFormat implementation. By fixing a common typo and improving error message readability, we enhance the overall code quality and developer experience.

Changes Made

• Fixed Typo: Corrected insures to ensures in the expect message within the ToLocalTime implementation in mod.rs.
• Improved Error Message: Updated the TypeError message in the get_format accessor in mod.rs. Changed initializedDateTimeFormat to initialized DateTimeFormat to make the error more readable for users.

Verification

• Unit Tests: Verified that existing Intl.DateTimeFormat tests still pass:
  • dtf_basic — Passed
  • supported_locales_of — Passed

[github-actions]

Test262 conformance changes

Test result	main count	PR count	difference
Total	52,963	52,963	0
Passed	49,687	49,687	0
Ignored	2,262	2,262	0
Failed	1,014	1,014	0
Panics	0	0	0
Conformance	93.81%	93.81%	0.00%

Tested main commit: b3d820369072123ded8defc927405b99b9012648
Tested PR commit: 5bf45be538963306c031c15d58acf73983228bdb
Compare commits: b3d8203...5bf45be

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 57.79%. Comparing base (6ddc2b4) to head (5bf45be).
⚠️ Report is 775 commits behind head on main.

Files with missing lines	Patch %	Lines
...e/engine/src/builtins/intl/date_time_format/mod.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4933       +/-   ##
===========================================
+ Coverage   47.24%   57.79%   +10.55%     
===========================================
  Files         476      556       +80     
  Lines       46892    60927    +14035     
===========================================
+ Hits        22154    35213    +13059     
- Misses      24738    25714      +976     

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