doc: Add missing 'blank=true' option in offline-signing-tutorial.md #31236

Issue:

The text mentions that the createwallet command should use the options disable_private_keys=true, blank=true, but the provided command only includes disable_private_keys=true, missing the blank=true option.

Details:

Original Text:

This is achieved by using the createwallet options: disable_private_keys=true, blank=true.

Original Command:

[online]$ ./build/src/bitcoin-cli -signet -named createwallet \
              wallet_name="watch_only_wallet" \
              disable_private_keys=true

Correction:

Added blank=true to the command to match the options described in the text.

Revised Command:

[online]$ ./build/src/bitcoin-cli -signet -named createwallet \
              wallet_name="watch_only_wallet" \
              disable_private_keys=true \
              blank=true

Explanation:

The blank=true option is necessary to create a blank wallet. Including this option ensures the command matches the options specified in the text.

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The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/31236.

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Please don't include the full diff in the pull request description. If someone wanted to look at the diff, they could just do so directly. Also, the diff in the pull description is different from the actual diff, which is broken (invalid syntax).