Add Express Configuration example to create-db-cluster CLI docs

[AndrewAsseily]

Summary

• Add a third example to awscli/examples/rds/create-db-cluster.rst demonstrating creation of an Aurora PostgreSQL DB cluster with Express Configuration enabled (--with-express-configuration).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.43%. Comparing base (c4681b5) to head (0783ced).
⚠️ Report is 344 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #10300      +/-   ##
===========================================
+ Coverage    93.39%   93.43%   +0.04%     
===========================================
  Files          210      211       +1     
  Lines        17052    17283     +231     
===========================================
+ Hits         15925    16149     +224     
- Misses        1127     1134       +7     

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