wallet: update walletpassphrase BGL help text

[JeremyZeng77]

Summary

• Updates wallet RPC help text to use BGL instead of stale bitcoin wording.
• Covers both walletpassphrase and encryptwallet help examples.

Verification

• git diff --check -- src/wallet/rpc/encrypt.cpp
• rg -n -- "sending bitcoin|sending bitcoins|sending BGL" src/wallet/rpc/encrypt.cpp

Related bounty: #32

Payout: USDT TRC20 TWupTAoFjxR2VmqUUvzhfmUwXVZR6Kvz1Y; PayPal https://paypal.me/Jeremytsangchina.

[MyTH-zyxeon]

Maintainer-facing review assist for the #32 Bitgesell PR bounty queue.

Current visible state:

• PR is OPEN / MERGEABLE / CLEAN at head e1e10a836150771733cf899d256d870d6c0fccf5.
• Diff is wallet RPC help/example text only in src/wallet/rpc/encrypt.cpp; no wallet locking, encryption, signing, transaction creation, consensus, or build path is changed.
• No prior comments, reviews, or visible status checks were present when checked.

Cross-check against master:

• walletpassphrase still says the command is needed before private-key actions such as sending bitcoins.
• encryptwallet still tells users to set the passphrase for signing or sending bitcoin.
• The repository already uses BGL-oriented naming in user-facing files and docs, so this aligns the wallet help text with the fork naming.

Suggested pre-merge check is just generated RPC help/example output for walletpassphrase and encryptwallet; there is no behavior change to validate beyond user-facing text.