Inconsistent use of `std::execution::get_stop_token` and `std::get_stop_token`

[lewissbaker]

There are several sections that refer to std::execution::get_stop_token() as the CPO to call to obtain a stoppable_token from an environment.

• https://cplusplus.github.io/sender-receiver/execution.html#design-sender-factory-read
• https://cplusplus.github.io/sender-receiver/execution.html#example-async-windows-socket-recv
• https://cplusplus.github.io/sender-receiver/execution.html#design-cancellation-summary
• https://cplusplus.github.io/sender-receiver/execution.html#design-lazy-algorithms-runtime

However, the wording currently has this CPO defined as std::get_stop_token().
See https://cplusplus.github.io/sender-receiver/execution.html#spec-execution.syn

We should make sure to be consistent between examples and wording.