Change Plugins docs to use correct framework monikers

[Itamaram]

I have changed the (incorrect) code snippets in the Plugins documentation page to match the (correct) text describing them.

I've created another PR in the samples project SpecFlowOSS/SpecFlow-Examples#68 to correct the same changes there too.

Types of changes

• Bug fix (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).
• Performance improvement
• Refactoring (so no functional change)
• Other (docs, build config, etc)

Checklist:

• [n/a] I've added tests for my code. (most of the time mandatory)
• [n/a] I have added an entry to the changelog. (mandatory)
• [n/a] My change requires a change to the documentation.
• I have updated the documentation accordingly.

[SabotageAndi]

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