[Cashtab] Airdrop text area header css patch

Summary: When user calculates an airdrop, the text 'one to many airdrop payment outputs' should be lightWhite, not black.

Test Plan:
cd web/cashtab && npm start
either from the airdrop tab, or by clickin the airdrop button in an eToken's info page, calculate an airdrop
observe that the text above the text area is lightWhite, not black

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11312