[wallet] Tidy up CWallet::FundTransaction

Summary:
Ended up being just a typo correction because of the linter.

Part 1 of 2 of Core PR11864
https://github.com/bitcoin/bitcoin/pull/11864/commits/95d4450a41e9b3f7a739eeefec322bf9366ce824

Commit:

[wallet] Tidy up CWallet::FundTransaction
95d4450a41e9b3f7a739eeefec322bf9366ce824

Test Plan:

make check
test_runner.py

Reviewers: jasonbcox, deadalnix, Fabien, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3192