fix: Fix broken nn.Flatten doc link in buildmodel_tutorial

[chinu0609]

Fixes #3948

Description

Update the nn.Flatten documentation link in beginner_source/basics/buildmodel_tutorial.py. The previous URL (https://pytorch.org/docs/stable/generated/torch.nn.Flatten.html) returns 404. Replace with the correct stable reference: https://docs.pytorch.org/docs/stable/generated/torch.nn.modules.flatten.Flatten.html.

Checklist

• The issue that is being fixed is referred in the description (see above "Fixes Feedback about Build the Neural Network #3948")
• Only one issue is addressed in this pull request
• Labels from the issue that this PR is fixing are added to this pull request
• No unnecessary issues are included into this pull request.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/tutorials/3951

• 📄 Preview Python docs built from this PR

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