Fix README immutable API examples

[dereuromark]

The README examples used Dto::create('Article')->immutable() which incorrectly calls the static factory method immutable(string $name) as a no-argument instance method.

The correct APIs are:

• Dto::immutable('Article') — static factory (used in all other docs)
• Dto::create('Article')->asImmutable() — instance method

This fixes the 3 occurrences in the Immutable DTOs section to use Dto::immutable('Name'), consistent with ConfigBuilder.md and all other documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.56%. Comparing base (4496cdb) to head (13c3f29).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #77   +/-   ##
=========================================
  Coverage     82.56%   82.56%           
  Complexity     1333     1333           
=========================================
  Files            40       40           
  Lines          3252     3252           
=========================================
  Hits           2685     2685           
  Misses          567      567           

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