Merge bitcoin/bitcoin#31237: doc: Add missing 'blank=true' option in …

…offline-signing-tutorial.md

ec375de doc: Add missing 'blank=true' option in offline-signing-tutorial.md (secp512k2)

Pull request description:

  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.

  Correction:

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

  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.

ACKs for top commit:
  fanquake:
    ACK ec375de

Tree-SHA512: 8c145e3ef1598c5e13f2aa89e496f76bfe2fc6f47d5e740963acad18dd1f782655a822dc234862af8e5a08060ab7fe1039a3dcfa68455a9143fe2d849975927c

doc/offline-signing-tutorial.md

@@ -60,7 +60,8 @@ The `watch_only_wallet` wallet will be used to track and validate incoming trans
 ```sh
 [online]$ ./build/src/bitcoin-cli -signet -named createwallet \
               wallet_name="watch_only_wallet" \
-              disable_private_keys=true
+              disable_private_keys=true \
+              blank=true
 
 {
   "name": "watch_only_wallet"