docs(netlify-example/brush/index.html): Fixes BrushTool example

[mikehazell]

Fixes a typo which throws an error in the BrushTool example

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Docs example - bug fix

To reproduce:

1. Go to https://cornerstone-tools.netlify.app/netlify-example/brush/
2. Inspecting the console will show this error

Uncaught ReferenceError: configuration is not defined
	at (index):542

3. The Active and Inactive alpha sliders have no effect.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  No

[codecov]

Codecov Report

Merging #1377 (c4248d3) into master (291b8e9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1377   +/-   ##
=======================================
  Coverage   18.93%   18.93%           
=======================================
  Files         286      286           
  Lines        8684     8684           
  Branches     1486     1486           
=======================================
  Hits         1644     1644           
  Misses       5827     5827           
  Partials     1213     1213           

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[dannyrb]

🎉 This PR is included in version 5.2.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀