docs(toolkit-lib): await cdk.fromAssemblyBuilder in readme

[kaizencc]

In subsequent examples we type cx as ICloudAssemblySource, so it makes more sense for our examples to show cx as an ICloudAssemblySource and not Promise<ICloudAssemblySource>.

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.05%. Comparing base (4408fb9) to head (906a407).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #258      +/-   ##
==========================================
- Coverage   85.28%   85.05%   -0.23%     
==========================================
  Files         218      218              
  Lines       36548    36548              
  Branches     4600     4564      -36     
==========================================
- Hits        31170    31087      -83     
- Misses       5232     5317      +85     
+ Partials      146      144       -2     

Flag	Coverage Δ
suite.unit	85.05% <ø> (-0.23%)	⬇️

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