doc: Switch boost::thread to std::thread in scheduler

Summary:

After core#18234 the scheduler itself no longer cares if the
serviceQueue is run in a std::thread or boost::thread. Change the
documentation to std::thread because we switched to C++11.

This is a backport of core#19090 [3/6]
https://github.com/bitcoin/bitcoin/pull/19090/commits/fab2950d703217ec34b27e677e4f33ebbf99ca08

Test Plan: Proofreading

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9957