Fixed wrong ELK detector

[nirname]

📑 Summary

Brief description about the content of your PR.

Resolves #5507

📏 Design Decisions

We have to check only the beginning of the line so we need grouping.
This

/^\s*flowchar|graph/.test("some graph")

was returning true, but it should not.

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 💻 have added necessary unit/e2e tests.
• 📓 have added documentation. Make sure MERMAID_RELEASE_VERSION is used for all new features.
• 🔖 targeted develop branch

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 5.73%. Comparing base (0044aa3) to head (66cd0b9).

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #5579   +/-   ##
=======================================
  Coverage     5.73%   5.73%           
=======================================
  Files          277     277           
  Lines        41988   41988           
  Branches       515     515           
=======================================
  Hits          2409    2409           
  Misses       39579   39579           

Flag	Coverage Δ
unit	5.73% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
packages/mermaid-flowchart-elk/src/detector.ts	0.00% <0.00%> (ø)

[varon]

Thanks for the fix - code LGTM.