fix: pass improve pluralization flag

[dpilch]

Description of changes

• pass improve pluralization flag (only affects Swift target)
• This flag was missed when adding the graphql-generator
• The flag overlaps with transformer version so customer are still able to activate the feature.

  amplify-codegen/packages/appsync-modelgen-plugin/src/visitors/appsync-swift-visitor.ts

  Line 394 in df65382

  const useImprovedPluralization = this.config.improvePluralization || (this.config.transformerVersion === 2);

• We will need to make a breaking change to graphql-generator to change the default to true.

Codegen Paramaters Changed or Added

improvePluralization is being respected.

Issue #, if available

N/A

Description of how you validated changes

• Unit testing

Checklist

• PR description included
• yarn test passes
• Tests are changed or added

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