[services] Tunnelbroker - Fix GetN()->GetS().

Summary:
This is a small fix error on std:stoull that throws an exception because it's expecting a string, but we return the number in GetN().
We've agreed before that we should use GetS() from AWS C++ SDK everywhere.
This diff changes GetN()->GetS().

Test Plan: Run yarn test-tunnelbroker-service and successfully build the app.

Reviewers: karol-bisztyga, palys-swm, varun

Reviewed By: karol-bisztyga, palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3642