[ecash-herald] Patch typo in secrets.sample.ts

Summary: We want to export the object, not the type

Test Plan:

cp secrets.sample.ts secrets.ts

update with values
node scripts/sendMsgByBlock and it works rather than throws type error

This error was not propagated in CI so it only impacts someone trying to run locally by following README instructions, or myself after accidentally deleting secrets.ts

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17089