fix typos in Flutter code examples for Modeling relationships documentation

[HyperToy]

Description of changes:

Fixed typos in Flutter code examples in the relationships documentation:

1. Lines 212-213: Fixed variable usage in "Update a Has Many relationship" example

   • Changed ModelMutations.create(team) to ModelMutations.create(newTeam)
   • Changed request: teamRequest to request: newTeamRequest
   • The variable newTeam is defined on line 211, so the previous references were incorrect

2. Line 562: Corrected variable reference in "Create a Has One relationship" example

   • Changed teamResponse.customer to customerResponse.customer
   • The variable customerResponse is defined on line 560, so teamResponse was incorrect

Related GitHub issue #, if available:

N/A

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• amplify-cli
• amplify-ui
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• JS
• Swift
• Android
• Flutter
• React Native

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