Fix wrong import name

[ghost]

Proposed changes

Fixing import name.
Because in the module purgess-webpack-plugin, the name export have been changed to PurgeCSSPlugin.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

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• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

Export name is PurgeCSSPlugin, not PurgecssPlugin.