Label Nethermind Private Payments link as Demo (#2558)

The nethermindeth.github.io/stellar-private-payments page is an interactive
testnet demo (Freighter connect + deposit/transfer/withdraw UI), not static
docs, so label it accordingly.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mmazco

docs/build/apps/privacy.mdx

@@ -37,7 +37,7 @@ Research prototype, not audited. Do not use in production with real assets.
 Proofs are generated client-side in the browser via WebAssembly — user secrets never leave the device.
 
 - **Repo**: [NethermindEth/stellar-private-payments](https://github.com/NethermindEth/stellar-private-payments)
-- **Docs**: [nethermindeth.github.io/stellar-private-payments](https://nethermindeth.github.io/stellar-private-payments/)
+- **Demo**: [nethermindeth.github.io/stellar-private-payments](https://nethermindeth.github.io/stellar-private-payments/)
 
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